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                Clear Space to Fix Catalina Update Restart Loop

   Adam Engst

   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 [1]some suggestions that too little free space could
   cause the installer to fail silently. I downloaded [2]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
   10.13 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.

References

   1. https://discussions.apple.com/thread/251230770?answerId=252788425022#252788425022
   2. http://grandperspectiv.sourceforge.net/

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