Update to original answer
I've tried looking through my own posts at forums and found out I actually have managed to find most of bloat left by unsuccessful update! Directories used by
softwareupdate tool and that never get purged are:
So you'll have tocheck and purge them using Terminal in macOS Recovery as well. Also regarding failed update error description mentioned in original answer's quote. I never found explanation besides
MCU 1130 Failed to seal system volume error in
/Volumes/Macintosh HD/Volumes/Update/last_update_result.plist. Perhaps Monterey got better in logging errors so it's worth checking this log file.
Check size of this folder:
Also check the actual free space of your Mac's main volume.
TL;DR your best bet will be to:
- Turn off automatic updates in
Settings > Software Update
- Delete contents of
com_apple_MobileAsset_MacSoftwareUpdate (if there are any and the size is significant), use Terminal in macOS Recovery with
rm -rf Volumes/Macintosh HD/System/Library/AssetsV2/com_apple_MobileAsset_MacSoftwareUpdate/<Random-ID> (edit x's to actual large folder names) but make sure to
diskutil mount diskXsX first (use
diskutil list to check volume's mount point)
- Free up space on main Macintosh HD volume to a minimum required disk space for fresh macOS Monterey installation which happens to be 44 GB
- Check for updates manually via
Settings > Software Update, download and run in manually, let update process run until it succeeds or an error/fail message appears (it can take 30-45 minutes easily even on SSD, don't pay attention to status indicator because it's lagging).
If you're extremely low on disk space you can try the steps with 26 GB of free space (this might work according to previous link). Hope this helps!
I'll keep the explanation brief - but believe me I struggled with
similar thing literally for weeks - and this might be situation
similar to yours. macOS's software update on Big Sur and up requires
a lot of free space to install update. And if update starts installing it may check available free space wrongly, as a result
update fails during installation but macOS snapshot prepared by
Software Update doesn't get deleted, as well as some update files (I
couldn't determine the location of that additional bloat of backups of
backups). I learned all that while tried updating clean macOS Big Sur
on a volume that was resized to smaller than minimum recommended size
(34 GB while 35.5 GB was a requirement) and I almost managed to
do this: my update process was starting and running for 99% making all
preparations despite the space requirement was not met, free space was
being filled with snapshots and other bloat, but 30 sec to the end
update process failed with literally no explanation. As the result only the firmware was updated. You can examine this post for log of my struggle.
Good thing to know once update succeeds it will most likely remove traces of unsuccessful attempts. However IMHO it's the best practice to write down disk space usage before checking for updates and installing them (since you can easily loose 10+GB of disk space for some update that fails silently).
Please comment if you'll achieve your goal!