"Other" storage suddenly huge and yes it is real loss of free space

Oh, and I should have added that booting into safe mode changed nothing. Not surprising I guess since all the normal things it checks (logs, cache etc) I already knew were not taking up the unknown space.

Thank you for that! I have GrandPerspective (had it for years) but I know it simply cannot access parts of the files system. So I have been plowing around in the CLI (which with sudo limitations in MacOS I find awkward to sometimes find hidden stuff - used to Linux and past UNIX's like IRIX where sudo=root and was genuinely God of all things.

I will check out those two and see what I can find with one of them. I did eject and shut down all external drives so I am just searching/querying the internal SSD.

Steve, thanks! I paid for DaisyDisk (at t$10 what did I have to loose) and got it sorted out. I still cannot say for certain what specific file was corrupt, but it was the purgeable space that was the issue. While MacOS should (and obviously does) handle that dynamically as disk space for user files is demanded, in my case it clearly was no longer doing so. When I had tried to copy a 42 GB folder to the drive from a thumb drive, that was when I got the error that there was insufficient space and reportedly I only had 30.8 GB available.

So, after deleting 635 GB of purgeable space, all is fine again. I assume it has to have been a corrupted Time Machine snapshot (although I understand Carbon Copy Cloner also makes snapshots, so maybe one of those). My cache and logs were all small so the system management of those areas was not the issue. Whatever it was, it was keeping the OS from dynamically managing that purgeable space so when I needed more space it could not clear it out for me.

OmniDiskSweeper does not even show any of that specific disk space usage, so wouldn't have helped me at all. GrandPerspective cannot access those hidden parts of the file system (hidden even to root in the CLI). The paid version of DaisyDisk not only can scan for those file system content, but has the ability to delete them to clean out the purgeable space when such can issue comes up. From what I have been able to discover, I think it is the only utility that can do so in Big Sur.

So I learned a few things about disk space management in MacOS Big Sur, and now have a handy (but hopefully rarely needed) tool to deal with such issues should then ever come up again.

P.Phillips - I appreciate the input, but the issue with that first link is it does not talk about the purgeable space that MacOS manages for things like Time Machine local snapshots. And those files are not remotely easy to find or access in Big Sur - they are all locked down, hidden and not even accessible (at least not by any means useable in any previous MacOS release) even to root.

Previous versions of MacOS and OS X, if you were comfortable using root, had ways to fix issues with most or many of those hidden system mangled files and directories. I could not find any way to do so with Big Sur though. I can see the snapshots in the CLI in Big Sur, but I cannot purge them as root.

If anything in that dynamically managed purgeable data gets corrupted such that MacOS itself can no longer manage it and release disk space as the user needs it, then you're stuck. At least stuck without a lot more system level admin knowledge than I possess for MacOS 11.6. DaisyDIsk is actually a brilliant solution to what for any user should be an unlikely, but obviously not impossible situation.

I have tried to keep up with all these sorts of changes but am definitely behind on my self-education. Now I need to see if/how to simply disable Time Machine (and CCC) local snapshots as I’ve done in the past with previous MacOS releases.

My Mac Mini is on 24/7 (the monitors sleep, but not the Mac itself) and it (the Mac not the monitors) and it’s TM and CCC drive are on an APC 1300 UPS which can easily power them all far longer than any power outage I’ve experienced in the past 5 years (and everything will get shutdown when the APC battery gets below 20% capacity). So I don’t even need local TM (or any snapshots) as my TM drive is never “not available”. So in the past just disabled snapshots.

So I now have a project to pursue I guess - how to disable TM snapshots in Big Sur (the older methods I know do not work due to the changes in the system). But at least even if I never figure that out, I have a way to clean things up should a snapshot ever get fubar’d again and the dynamic disk space allocation gets locked up.

And I did submit feedback about this experience as it would be good for Apple to proactively provide some means of force-purging purgeable space in the rare event the OS itself gets stuck doing it because of some corrupted file or directory. Maybe a “rescue purgeable space” option in disk manager or something like that?