DISCLAIMER: I haven't done enough research to determine if this is a bug, an isolated issue with my install, etc.
I foolishly let my main drive fill up 100% (I was rsync'ing files to my computer while gone). When I got back it was super slow and I restarted it (not knowing it was full) to see if that would speed it up. Lo and behold, the darn thing wouldn't boot.
I pulled the drive and mounted it on my MacBook via SATA to USB and I realized the drive was 100% full. Figured no big deal; I'll just delete some stuff. NOPE. I'm not sure if this is only on APFS, or on HFS too (no HFS devices left to check on), but it seems that APFS has COW (copy-on-write) which is a feature also found on ZFS. I won't pretend to understand everything about how it works, but essentially, using the "rm" command doesn't work; you are supposed to "clobber" the file by using "echo > /some/file/to/delete." Unfortunately, this didn't work. I tried all the fixes for ZFS filesystems that I gleaned from Google (dd, truncate, etc) but nothing would work. Basically, APFS seems to need space to delete something no matter what.
The only way I was able to fix it was by removing my Recovery partition using "diskutil apfs deleteVolume <MY VOLUME>." I can't use it anyway, so hopefully it's not a big deal.
Maybe someone with for filesystem knowledge can shed some light on this, but I would at least be cautious about letting your drive fill up (it's bad for performance anyway). I also wonder if this has anything to do with it being a hackintosh.
TL;DR Filling up the drive leads to a catch-22 where you can't delete files to regain space; maybe just isolated bug.
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