I haven’t been using my old 2012 MacBook Air recently, so I wasn’t surprised when System Preferences displayed a badge alerting me to updates. It wanted to install Security Update 2021-002 for macOS 10.15 Catalina and Safari 14.1, so I clicked the Update Now button and received a dialog telling me that I needed to restart to complete the update. No problem, so I clicked Restart.
The MacBook Air promptly restarted, and I logged in again, only to discover that nothing had been installed. I tried again, and once again, the Mac just restarted without installing the updates. I switched to a different admin account, but the problem recurred, all without any error dialogs that might have shed some light on the problem.
On a hunch, I checked the free space available on the MacBook Air’s internal 256 GB SSD. It was about 7 GB, which didn’t seem ridiculously low for just a security update, but after some hunting around on the Internet, I found some suggestions that too little free space could cause the installer to fail silently. I downloaded GrandPerspective, my favorite utility for identifying large files, cleared 40 GB, and tried again. This time the update installed as it was supposed to.
The moral of the story is to make sure you have sufficient free space before installing even a security update. What’s sufficient? That’s unclear, though 47 GB was enough, and some of the comments suggest that 20–30 GB would work.
Of course, the larger lesson is that it’s not a good idea to let a Mac’s internal drive get too full. If you search, you’ll find recommendations ranging from 5% to 20%, comments suggesting that percentages don’t make much sense for larger drives, and responses pointing out that it all depends on what you’re doing. Clearly, 7 GB free on a 256 GB SSD (2.7%) wasn’t enough for an installation to proceed, whereas 47 GB (18%) was. We also know that if you want to upgrade to macOS 11 Big Sur, it needs at least 35.5 GB to upgrade from 10.12 Sierra or later and 44.5 GB to upgrade from earlier versions of macOS.
So I’d suggest that 50 GB may be a safe minimum for most Macs. More is always better due to virtual memory swap files, the need for large caches, and APFS snapshots.
Have you had trouble after running low on free space? Share your experiences in the comments.