This article originally appeared in TidBITS on 2013-03-13 at 1:41 p.m.
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FlippedBITS: Booting Your Mac from a Duplicate

by Joe Kissell

In this first installment of FlippedBITS, I want to look at what happens when you boot your Mac from a duplicate (or “clone”) of your startup disk. In doing so, I hope to clear up several common points of confusion, particularly regarding ongoing backups and syncing other types of data.

For years I’ve recommended a three-pronged backup strategy consisting of versioned backups (such as those produced by Time Machine or CrashPlan), bootable duplicates (complete copies of everything on your startup disk, stored on an external drive), and offsite data storage (either in the cloud or by rotating physical media to other locations). Together, this combination can protect your data against almost any disaster, while making recovery as painless as possible. (For complete details about my suggested strategy, including the steps to create a bootable duplicate, see “Take Control of Backing Up Your Mac [1].”)

It’s simple enough to make a duplicate using a tool such as Carbon Copy Cloner [2] or SuperDuper [3]; having done that, you can use the duplicate to boot your Mac either by selecting it in the Startup Disk pane of System Preferences or by holding down Option while restarting and selecting the volume containing the duplicate. Doing so enables you to get back to work immediately if anything goes wrong with your startup disk; running from the duplicate makes your Mac behave as though nothing had happened. So far, so good.

But, based on numerous email exchanges I’ve had with people who have read my various books and articles about backups, what happens once you’ve booted from the duplicate is sometimes unclear. Some people expect a duplicate to behave entirely like the original in every situation, which turns out not to be quite true. Others worry that the duplicate will cause all sorts of problems because it’s not enough like the original, resulting in extra, unnecessary steps. To untangle things, let’s start with the least ambiguous situation.

Swapping Your Startup Drive -- Suppose your Mac’s internal hard drive dies completely, so you remove it from your Mac and replace it with the drive on which you’d previously stored your bootable duplicate. Your Mac doesn’t care about the fact that the new hard drive may have a different brand, capacity, or speed. All it knows is: here’s a disk with exactly the same data in exactly the same place. It is, for all practical purposes, the same disk. You can carry on as if nothing happened; everything, including your backups, should simply pick up where they left off, which is almost certainly what you want.

Now, there is one little catch. What if there was a time lag between when you made the duplicate and when you started using it? And what if, during that lag, you created or edited data on your startup disk — and backed it up to a destination other than where your bootable duplicate is stored? Now you have a startup disk that’s somewhat out of date, and to make it current, you’ll have to go to that other backup to locate and restore any important files that were changed after you last updated the duplicate. This, I’m sorry to say, is often an entirely manual procedure. In many backup apps (and again, I’m thinking especially of Time Machine and CrashPlan), there’s no simple way to say, “Show me all and only the files that changed and were backed up after time x.” You may have to dig through folders one by one in your backup archive to find these files. You could ask the software to restore everything backed up after a certain time, overwriting any existing files, but that would take quite a while. It’s a pity that many otherwise highly competent backup apps don’t account for this usage case.

Booting from an External Drive -- Although the situation I just described is the least ambiguous, it’s also relatively infrequent. The most likely scenario, which gets more confusing, is when your regular startup disk is still present and functional but you hook up your duplicate and boot from it temporarily. Perhaps you’re doing this to verify that the duplicate works (in which case you may be running from the duplicate for only a few minutes), or perhaps your startup disk is having problems and you want to run a disk repair utility while carrying on with your regular work.

Either way, let me start by saying what isn’t a problem in this scenario. For one thing, it doesn’t matter if your startup disk is external. Apart from speed differences, your Mac should behave identically whether the startup disk is connected via an internal SATA port, USB, FireWire, Thunderbolt, or whatever. So, don’t let that trouble you in the least.

For another thing, it doesn’t matter if data happens to sync with the cloud while you’re booted from the duplicate. For example, if you use iCloud, your calendars, contacts, bookmarks, and so on will sync in the background. You need not worry that the outdated data already on your duplicate will somehow overwrite what’s in the cloud; on the contrary, the cloud has the “master” copy (sometimes called the “truth”), so it will bring the data on your duplicate disk up to date. Similarly, if you use Dropbox or another cloud-based file storage service, it will bring your disk up to date with the latest truth from the cloud, and it’s unnecessary for you to fret over that in the slightest.

(You do need to fret if you use POP for email, or if you have any rules or filters that file incoming email from IMAP or Exchange servers into local mailboxes. That could get messy, with the duplicate being changed in ways that can’t easily be applied back to the original, so if in doubt, refrain from checking your email at all while booted from the duplicate.)

You need not even worry about aliases — usually. When you create an alias to an item that’s on your current boot drive (that includes items in the Finder’s sidebar and in your Login Items list), Mac OS X creates relative links. That means if you make a duplicate, boot from the duplicate, and open an alias, the item that opens is the one on the duplicate, not on the original disk. (And that’s probably what you want.) That’s not to say your Mac might not have a script, a symbolic link you created in Terminal, or some other pointer that references a file or folder by disk name, and if it does, you could accidentally open the wrong copy of a file or application, or save data to the wrong disk. (If you’re concerned and want to be absolutely sure which item you’re opening, navigate manually from the top level of your disk when booted from a duplicate.)

However, at least one significant thing is most likely different, even though you may not notice it. If your duplicate had the same name as your startup disk (presumably the most common case), something slightly weird can occasionally happen. Mac OS X won’t let two mounted volumes have exactly the same name. In the Finder, they may look like they have the same name, but if you already have a volume called “Macintosh HD” mounted and then you mount a second one, behind the scenes, the second one gets a different working name (in this case, “Macintosh HD 1”). That’s because many things that happen on your Mac depend on being able to locate a disk by name, and if there were any ambiguity, a file might get put in the wrong place.

Ordinarily, this on-the-fly renaming just works, but it’s not foolproof. What if, during the time you have both “Macintosh HD” and “Macintosh HD 1” mounted, another user on your network connects to your Mac and copies a file to what is now “Macintosh HD” — your duplicate? You might not notice it, and when you switch back to your usual startup disk, the file would be missing. Similar things can happen with file-synchronization apps, software downloads, and other operations. Furthermore, sometimes Mac OS X gets confused and doesn’t correctly update its behind-the-scenes list of volume names, so you could, for example, encounter a situation in which “Macintosh HD” is a volume you mounted after “Macintosh HD 1.”

On the other hand, renaming your duplicate doesn’t necessarily solve these problems. If your normal startup disk is named “Cindy” but you’ve booted temporarily from “Kate,” you may avoid mismatched name issues right now — but later, if you have to start using “Kate” permanently, apps and users that were still trying to save data to “Cindy” could get confused. All in all, I think you’ll get the best results if your duplicate has the same name as the original disk, but you should follow a few steps (just ahead) to avoid problems while running from the duplicate.

Meanwhile, you may have to think about another subtle background process: backups! After all, your backup software is probably configured to run automatically — perhaps once an hour (like Time Machine) or continuously (like CrashPlan). Backups usually don’t begin immediately when you boot your Mac, but they could easily kick in within 10 or 15 minutes. What if you’re planning to run your Mac from the duplicate for longer than that, but not permanently? Should you let your backups proceed — meaning they’ll be backing up the duplicate — or should you turn them off?

In general, there’s no harm, and considerable benefit, in letting backups run. Your backup software should act as though your duplicate is your regular startup disk and keep copying files to its normal destination as though you had restarted normally. That’s probably what you want, because if you create or modify a file while running from the duplicate, it can then be backed up. The problem is, in fact, with the opposite case: what if you modify a file but it isn’t backed up (perhaps because the time for the next periodic backup run hasn’t rolled around yet)? When you switch back to your regular startup disk, the file won’t be there (or won’t be current), and it won’t be in your backups either. It will still be on your duplicate — but only if you think to check there before you update your duplicate the next time; doing so will probably delete the new file because it’s not on your startup disk.

Taking all this into account, here are my recommendations for what to do when you must boot from a duplicate for a short period of time:

Booting a Different Mac -- There’s one more scenario to consider: booting one Mac with a duplicate of another Mac’s startup disk. For example, imagine that you created a duplicate of your iMac’s startup disk and then you had to take your iMac to the shop for repairs. In the meantime, you hook up your duplicate to your MacBook Pro so it can “pretend” to be the iMac. As long as the MacBook Pro supports the same version of Mac OS X your iMac was running, this arrangement should work fine — with, as you might have guessed, a couple of qualifications.

First, although this situation is ostensibly similar to the last one — booting a Mac temporarily from another drive — the time frame involved could be longer (days or weeks instead of hours). So, it’s impractical to avoid modifying files, checking your email, and the like. Therefore, I recommend that you use the duplicate disk on your MacBook Pro as you normally would use the internal drive on your iMac, and then — once your regular iMac is back in service — hook up the external duplicate and reverse the cloning process (copy everything from the external drive back to the iMac’s internal drive it started on).

Second, while your MacBook Pro is running from the duplicate, it will by almost every measure appear to be the iMac. It’ll even use the iMac’s name for file sharing, screen sharing, and the like. However, every Mac has several unique identifiers, including a serial number, a UUID (universally unique identifier), and a MAC (media access control) address for each network interface. Some pieces of software check one or more of these unique identifiers to verify that they’re still running on the same Mac on which they were authorized or licensed.

The most common example is iTunes. If you authorize your iMac to use your iTunes account and then start up your MacBook Pro with a duplicate of the iMac’s disk, that doesn’t mean the MacBook Pro is automatically authorized. You must authorize it manually, if you haven’t done so previously (in iTunes, choose Store > Authorize This Computer). But, if you’ve already run out of authorizations — Apple limits you to five — you may be out of luck unless you can deauthorize one of your other computers or reset all your authorizations [4], the latter being something that Apple allows you to do only once per year. Other software may make you jump through similar (or worse) hoops.

Don’t Sweat It -- As convoluted as this all may sound, booting your Mac from a duplicate is usually a simple and problem-free operation. But it never hurts to have a better grip on what’s going on behind the scenes, just in case. In particular, think about whether you’re going to be using the duplicate long enough to add or modify files on it manually. If not, there’s no harm in simply switching back to the original. But if you are going to be working from the duplicate for any significant period of time, you’ll need to clone it back to the original when you’re done.

A final note: keep those duplicates up to date. It would be overkill to update them every hour, but once or twice a day is not a bad idea. Remember, the longer the gap between the last time you updated your duplicate and when you discovered you needed to boot from it, the greater the chance of missing or outdated files that you may have to laboriously restore.

[1]: http://www.takecontrolbooks.com/backing-up?pt=TB1165
[2]: http://bombich.com/
[3]: http://www.shirt-pocket.com/SuperDuper/SuperDuperDescription.html
[4]: http://support.apple.com/kb/ht1420