Shortly after installing macOS 26.1 (update from 15.7.1), apps report no disk space available (for instance Mail: there isn't enough space in your home folder), or just refuse disc interaction (can't save files anywhere). Finder shows lots of available space, but can't use it (when creating new folder: the disk … is full).
Mac mini, macOS 26.1
Check Disk Utility or the System Report to see free space. Finder shows 'available' space and that's not the same thing.
See this User Tip:
How to free up ‘System Data’ and other st… - Apple Community
Check Disk Utility or the System Report to see free space. Finder shows 'available' space and that's not the same thing.
See this User Tip:
How to free up ‘System Data’ and other st… - Apple Community
I had been getting that message you see from Mail a couple of macOS versions ago. I could temporarily clear the problem by deleting the local Time Machine snapshots, but the message eventually came back. I ended up reinstalling macOS and never got the message again. I do not remember if I erased all content in settings or just reinstall macOS. I had never seen any other app problems as you describe, though.
I know about the free space display challenges, but every disk is suddenly full at the same time, not even room for apps to change preferences. Sometimes after rebooting I'm able to use disks for a while, but then they apparently run out of spacce again. It even goes for the iCloud drive, which I hardly ever use (manually)
I ran Disk Utility First Aid on the startup volume, no problems there
The problem suddenly disappeared after about a week. No idea what happened, and of course I don't know if it might come back. Thanks for all suggestions!
Glad it's sorted itself out!
Mac mini M4 unable to use disk space after macOS 26.1 update
