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Ensure Sufficient Free Space before Upgrading to Big Sur

[Just before we published this article in TidBITS #1550, Mr. Macintosh reported that Apple has released a macOS 11.2.1 Big Sur installer that properly checks for sufficient free space before proceeding to avoid the problem described below. But you should still make sure you have enough space and a good backup before upgrading! –Adam]

As we continue to use macOS 11.2 Big Sur, we’re getting closer to saying that it’s safe for most people to upgrade to Big Sur from a previous version of macOS. That’s not to say that some people won’t have problems—no app or operating system is free of bugs—but most reproducible problems should be known.

Alas, one more such problem has become known, thanks to Mr. Macintosh’s detailed research into problems with the Big Sur installer. It turns out that you must have sufficient free space for the Big Sur installer to complete its work when upgrading from a previous version of macOS. If you start the installation without enough free space, you can end up in a Boot Recovery Assistant loop that’s difficult to break.

The problem affects only upgrades from an older version of macOS to Big Sur, not an update to a Mac already running some version of macOS 11. It also affects only Intel-based Macs; since M1-based Macs ship with Big Sur, they’re never in a position of having to upgrade from an earlier version of macOS.

Apple is clear about how much space Big Sur requires, specifying it in the Big Sur tech specs:

If upgrading from macOS Sierra or later, macOS Big Sur requires 35.5GB of available storage to upgrade. If upgrading from an earlier release, macOS Big Sur requires up to 44.5GB of available storage.

You can check manually by choosing Apple > About This Mac > Storage, but the problem is that the Big Sur installer itself fails to verify that it has enough space to complete the upgrade. That would seem like an easy fix, but Apple apparently hasn’t addressed it yet. If you’re perturbed by this lapse, submit feedback to Apple.

Free space report in About This Mac

If you or someone you know ends up in the Boot Recovery Assistant loop after attempting to upgrade to Big Sur, there are various workarounds, depending on whether or not your Mac has a T2 chip and whether or not you have enabled FileVault. Of course, everything is easier if you have a backup, and you should always make a backup before upgrading your Mac. When that’s the case, you can just boot to macOS Recovery, erase the drive, reinstall macOS, and restore from backup.

Since we assume all TidBITS readers know enough to maintain regularly updated backups and would never consider initiating an operating system upgrade without at least one current backup, let’s say that you’re helping someone else who started the Big Sur upgrade without sufficient space or a backup. The goal here is to remove extra files (I’d aim for a few large apps that are easily reinstalled):

  • FileVault disabled: If FileVault isn’t enabled, Mr. Macintosh outlines three options:
    • Boot into macOS Recovery and choose Utilities > Terminal to launch Terminal so you can remove files with the rm command. Be careful when deleting from the command line!
    • For Macs without a T2 chip, you should be able to boot from an external hard drive or a USB flash drive and then delete files in the Finder once the Mac is running. Be sure to empty the trash.
    • If you have a second Mac and appropriate cabling, connect the two Macs, boot the troubled Mac into Target Disk Mode, and then delete files from that Mac’s internal drive using the Finder on the host Mac. Again, be sure to empty the trash.
  • FileVault enabled: For those with FileVault enabled, Mr. Macintosh says that the Target Disk Mode approach is the only one that will work, with the caveat that the host Mac must be running 10.13 High Sierra or 10.14 Mojave. I don’t fully understand his explanation here, but it seems to revolve around how Catalina and Big Sur prompt for the account password before entering macOS Recovery, whereas the older macOS versions prompt when the troubled Mac in Target Disk Mode is connected.

Although this problem is unusual, I’ve received reports from several Apple consultants whose clients have found themselves in such a situation.

In the end, the morals of the story are to check for sufficient free space before upgrading to Big Sur and to maintain good backups, just as Joe Kissell recommends in Take Control of Big Sur. Nevertheless, kudos to Mr. Macintosh for the detailed testing and troubleshooting that was required to track down this issue.

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Comments About Ensure Sufficient Free Space before Upgrading to Big Sur

Notable Replies

  1. The case could be made that the paltry, default storage included in newer Macs is at least partly responsible for this issue, especially the 256GB option.Users can quickly fill up storage with photos, videos, music, documents, etc. Then when they update macOs they don’t have enough spare room for the process.

  2. Sure, though if Apple increased the cost of the base level Mac by $200 to match with a 512 GB default storage level, people would complain about that too. Let’s stay focused on the issue of the installer and recovering from the problem.

  3. Any Mac’s storage can be filled by media. It’s not just people who choose small size SSDs.

    How many downloaded movies do you think it would take to fill even a 2TB volume?

    Ultimately, Apple needs to implement one fix, and I would like to suggest a second:

    1. The upgrade installer must have an accurate estimate of required storage. If it’s not available, it must refuse to run. This is almost certainly a bug, since other upgrade installers do this, but there’s no excuse for letting this slip through the cracks.

    2. I think Apple should allow installation from external media. If your internal SSD is full, you should be able to copy the installer to external media (flash drive, SSD, hard drive, etc.) and run it from there. When run in this capacity, it should use the external media for its temporary file storage. This will (hopefully) limit the free space required on the internal volume to that needed to hold the upgraded installation and little more than that.

  4. Sorry Adam. Hit a sore spot. Anyway, did have an issue with 2016MBA that ran out of space with latest Big Sur update. After spending a few hours backing up, finding snapshots from old TM issue, the owner decided to “buy new MacBookAir” with ample storage.

  5. Without trying to argue the merits of Apple’s decision, here are some possibly interesting data points:

    • Cheap PCs also ship with configurations I would consider below the point of being useless. For example, the least expensive Windows 10 PC on WalMart’s web site is a $200 Gateway GWTC116-1, which only includes 4GB RAM and 64 GB storage. And the storage is eMMC - effectively a soldered-down SD card.

      I realize that Apple doesn’t sell anything at the $200 price point. My point is simply that least-cost options from all brands are going to be stripped-down, and the PC world has examples that are far more useless than anything Apple sells.

    • My current personal laptop (a 2011 MacBook Air) has only 4GB RAM and 128 GB storage. It is running macOS 10.12 (Sierra). For what I use it for (general web surfing and remotely logging into my other computers around the house), it works fine. The storage is not a problem for me because it connects to my home’s server (which has 2TB of storage) for accessing my media collections.

      Although I wouldn’t want this configuration on my main/only computer, I also wouldn’t want to spend more for a computer used in this capacity.

    • Apple probably should publish something like a buyer’s guide with recommendations for how much RAM/storage/CPU is appropriate for different use-cases. For instance:

      • Minimal configuration for web browsing, streaming, office applications, and only light multitasking (not running multiple large apps at once)
      • Larger configuration(s) for content creation, more multitasking
      • Extra storage to store media collections. Include some typical sizes (e.g. how many songs/movies can you expect to store in different capacities.) Similar to how they used to advertise iPod capacities. And include a comment about external vs. internal storage.

    Maybe we should prepare such a buyer’s guide if Apple doesn’t. :slight_smile:

  6. Seeing that Apple lacks the integrated warning/limit controls that Windows has, and searching for what is taking up space can be frustrating for casual mac user, I typically use OmniDiskSweeper to look at what is taking up space. But as David C. said, Apple really needs a feature/questioneer/guide for approximating your use, storage and capacity.
    Moving to Big Sur, and having a mac that is more than 5 years old, might quantify beyond just figuring out what apps need updating (and costs). Storage is exponential in OS progression, updates (security and OS), and unforseen increases with multiple users and syncing devices (iphone backups, …).
    Most do not have home servers, and Apple’s targeted users spend money to save to the cloud (photos, Music, …). Many don’t even understand Time Machine and the need to have an external (even setting iCal to remind to connect the USB drive and do a backup). With a portable and depending on Snapshots can take the user’s space up. And I’ve had some experience with Apple computers and the user writing more than the drive can store (e.g. friend’s daughter at school, saving files, didn’t know it was past the cap…till too late— Apple needs an integral storage watchdog that Notifies 20% remaining, 10% and finally stop the user from storing locally until space is made. Lower than 10GB means problems to the OS, caching, etc.

  7. CleanMyMac X has a nice feature for alerting you when free space drops below a certain amount or when the trash gets too big.

    But I wonder if there’s a way to do this in macOS itself. This discussion has various hints that suggest there may be a way to set default write options for to do this. Or it may have gone away in a more recent release. Anyone want to explore for us and report back?

  8. In Big Sur, the Storage Tab on the ‘About This Mac’ window can provide useful tools vis the ‘Manage’ button for working with storage on the boot drive. It brings up the ‘Storage Management’ window form the System Information app. If you click the ‘Review Files’ button, you will be presented with a file browser that initially shows the files in the Documents folder (including inside its subfolders) ordered by size. You can switch the folder used via the Sidebar. From the tab bar, you can also specifically see and delete files inn the Downloads folder. The ‘Unsupported apps’ tab shows 32-bit apps from the apps folder. It’s easy to select them and delete them quickly here.

    Since I didn’t even look at this before Big Sur, I don’t know if there is a similar utility buried in Catalina and earlier OS versions., but it appears to be an efficient way to deal with candidates for removal if space is tight.

  9. The Storage Management feature has been there since Sierra, so most people who are going to upgrade to Big Sur should be able to use it. It’s good, though I like GrandPerspective and Josh is fond of DaisyDisk for finding large files to remove.

  10. Sierra has this too, never knew that. What does “Optimize” do? Does it do something automatically as soon as you click it or are there other options?


  11. The article linked above should explain, no?

  12. Ah! Probably. I am a few days behind on emails and apparently missed the prior one - sorry about that!

    An Embarrassed Diane

  13. Excellent, if awkward timing for us. I’ll add a note at the top of the article.

  14. Regarding your note about the importance of backing up before doing upgrades (yes!) there is another important point that should be noted here.

    The venerable SuperDuper! backup program does not work with Big Sur. The folks from shirt pocket software are well aware of the issue and have been since about November. They report it is due to a change in Big Sur that makes it virtually impossible to make a full backup of a boot volume. (It apparently makes it literally impossible on Macs using the new chip.)

    You can back up external non-boot volumes, but those who rely on SuperDuper! should look into this before upgrading!

  15. As I understand it, you should still be able to install Big Sur to an external device and then use SuperDuper! to clone your data volume to that device’s data volume.

    The result will be a bootable system, but without the latest system software updates (unless you periodically boot from it and install macOS updates from there). That might be good enough until SD gets updated.

    Or you can switch over to CCC. According to Mike Bombich, Apple bundles an APFS replication utility which will properly clone the system volume on Intel Macs, and the latest versions of CCC use it.

    The only problem there is if you have an M1 Mac. The Apple utility currently has a bug preventing it from working on M1 Macs, so CCC can’t clone the system volume on one. I do expect Apple to fix this bug (although it’s been several months since it was reported to them) because Apple supposedly uses this utility in-house for pre-loading the factory software image, but who knows when they’ll decide to release a working version for the rest of us.

  16. That is correct, but for those of us who set up our primary automated backups via SuperDuper! the outcome has been quite messy, and the solutions are (still) not great.

    In my case, I had SuperDuper! doing automated daily backups to two volumes. One contained the backup of my system volume and the other contained the backup of my external (photography) files volume.

    After updating the operating system, my SD backups started failing and my computer was crashing in the middle of the night while trying to do the backups. Once I figured this out (Shirtpocket, usually a great company, never notified us) I had to turn off the boot volume backups so that things would not crash, causing the external data volume backup to crash.

    In order to do what you describe with SD! you have to uninstall your current version of SD!, download and manually install an older, deprecated version of the application, re-enter license information, replace the old boot volume backup (likely after reformatting the backup volume), and create a new one that only backs up the hidden data volume of the boot drive.

    It isn’t a totally non trivial process.

    I wish that Shirtpocket had used their email database to communicate with us about this. I’ve only managed to figure this out by tracking down a blog operated by the owner of the company — it is not part of the Shirtpocket website. I sympathize with them since it seems like Apple may not have given warning. I think Apple deserves some blame here, too


  17. I’m not familiar with SD, but this seems like more work than should be necessary.

    You should (I hope) be able to configure SD to only backup the data volume of your system disk. Then you should be able to download and run the Big Sur installer to install itself to your backup drive, which will create/update its system volume, making it bootable.

    It’s awkward, but it may be a good enough workaround for now.

  18. Please read Dave Nanian’s blog item for the explanation of why you need to go through the full process to essentially do that in SuperDuper!. Essentially, the few versions of SuperDuper! arranged all sorts of permissions and clever programming to work with the various issues Apple created in securing the system portion of the boot drive and keeping up the fiction to a user that nothing had changed. Multiple steps are needed to unwind the tricks and get back to a version of SuperDuper that will create a backup that is usable for restoring old data or migrating old data to a new system. The one thing that version can’t do is create a full bootable disk.

    Dollar to donuts, Nanian will release a version of SuperDuper! that performs the conversion steps the first time it’s run. However, given that Big Sur has been out for several months, he wanted to provide a way for his users to keep running and still have a reliable backup without waiting for him to release a slick version that really is only different from the older version the first time it is run.

  19. Yes, the “insufficient space” problem (bug, dammit!) is ridiculous, and the fact that it’s been corrected is laudable (but should never have happened in the first place; I wonder how seriously Apple takes quality control nowadays). But an even bigger and related problem is that Apple has made bootable clones impossible (and along with that, making repopulation of the Mac’s drive with the clone impossible, too). Dave Nanian (SD), Mike Bombich (CCC) and others (TTPro) may be able to successfully re-enable this, but there’s no guarantee.

    But I put the blame for this squarely on Apple. If Nanian et al succeed, then Apple should have been able to allow it, too, from the get-go. And if they don’t succeed, Apple should themselves correct the situation. In any case, it demands correction, because this capability is necessary to be able to quickly and efficiently recover from any really bad situation which may occur (like this space problem). I’m not sufficiently tech-savvy to know why Apple did it, or felt they had to do it (security?), but I can’t imagine that it absolutely needs to be that way.

  20. Looks like the latest version removed a feature (the ability to see and back up the data volume independently from the system volume) that the workaround depends on. I wonder why he can’t put that feature back, but maybe the code’s organization would make that too difficult to do without unwinding other recent features.

    I’m not sure why he’s got this phobia against using ASR to back up the system volume, which everyone seems to agree works fine on Intel Macs even though it is currently broken on M1 Macs.

  21. Here is Carbon Copy Cloner’s approach, which is now clear after Big Sur has gone through some system updates:

    If the system volume has not been updated, CCC does an incremental update to the data volume clone, preserving the boot status of the backup. However, if the system volume changes, CCC offers the user a choice of just updating the data volume clone or rebuilding the whole boot volume. Updating only the data volume destroys the ability to boot from the clone while allowing it to be a source for migration after a new system is installed. Rebuilding the whole clone preserves the ability to boot but takes longer.

    For example, I create a clone nightly from my iMac 2TB internal SSD to an external USB hard drive. When there is no change to the system, it generally copies 10-15GB of data in 30-40 minutes. However, when there was a system upgrade (from 11.1 to 11.2 and again from 11.2 to 11.2.1), the full run took 2.25 hours to clone 1.1TB of data.

  22. Too bad Apple can’t count. We cleaned the drive so 32GB was free, then this. Finder thinks 31.9GB available. Installer thinks 26.5GB is available. How hard can this be Apple.

    This is the .1 installer obviously since it gives the warning. Updating from Catalina. Was waiting for things to settle out since no to us important new features.
  23. Actually, determining exact amounts of free space can be quite difficult these days. For instance, with APFS, copies of files may be able to share storage with the original. Plus, with privacy sandboxing of folders, data in those folders can be hidden such that nothing can figure out how big they are.

    Now, that’s not to excuse Apple from creating a system where the Finder and the macOS Installer agree on how much space is free, but just to note it’s not a trivial problem.

  24. I don’t intend to pile on here. Clearly this is an Apple blunder.

    Nevertheless, I wanted to point out that it’s always been considered hazardous to run disks so close to full capacity. SSDs have changed that somewhat compared to the old HDDs because of the way they write data and their built-in housekeeping, but also because of how it affects their performance compared to a HDD. That all said, it remains true that running a disk (SSD or HDD) almost completely filled up is a disaster waiting to happen. In the old days, the rule of thumb was never below 10% unused, 20% was better. Even today with SSDs, if you have a 500GB or larger disk, IMHO you should really try freeing up more than just a paltry 32GB.

  25. I was about to suggest this is another case of power-of-10 units vs power-of-2 units, but that doesn’t add up to your observation:

    32 GB = 32,000,000,000 bytes = 29.8 GiB
    32 GiB = 34,359,738,368 bytes = 34.4 GB

  26. Another apparent upside to having more space is speed. It was my wife’s MBP and after Big Sur install, now has about 50GB on 512GB drive and she reports that her machine is much more responsive. Could be the usual it’s faster because it’s new; but I’m inclined to believe her impression.

  27. Yes, sufficient free space will let virtual memory swap to disk better, so it will provide better performance. I suspect that operating system developers don’t put a huge amount of effort into improving performance in highly constrained resources scenarios anymore simply because resources like memory are so cheap to expand.

  28. Coming in late on this because I just got bit by the bug on my circa 2018 MacBookAir with 128 gig of storage and it is now crawling its way through a Time Machine backup of the previous Mojave system. I did get a warning that I needed to clear more memory, but I may not have succeeded. I cleaned out iCloud Drive so it shows 14.13 Gbytes on Get Info my macMini, but Storage under “On This Mac” shows 29.8 Gbytes. That’s a rather large inconsistency, and may have tripped up the Big Sur install.

    I bought the Air on sale a couple of years ago, and it is essentially a backup machine which I mainly use when I want a computer outside my office. I also use it as a test machine for debugging, and in this case to test if it’s safe to upgrade the MacMini from Mojave to Big Sur. Now that I have established that the short-term answer is NO, I’m wondering where to go from here.

    After recovery, On My Mac tells me the Air has only 6 Gig of 121 gig available, with 72 Gig of “Other volumes in Container,” numbers which seem to be fluctuating as the Air sorts through the recovery. Do I need 32 Gig for recovery? How can I tell what the “other volumes” are? I don’t have many non-Apple apps or documents on the machine, and I’m wondering if it’s possible to strip it down enough to upgrade without making it unusable?

    Would trying a Catalina upgrade first do any good?

    What does it tell me about the prospects for upgrading the Mini to Big Sur? I am a heavy Apple Mail user, and from all that I have read about the problems with the Catalina version of Mail, it sounds that I don’t want to go to Catalina unless I immediately upgrade to Big Sur?

    FWIW, I did read Take Control of Big Sur, although it’s entirely possible I missed something.

    Thanks for your help,

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