Scratch Disks Full when they really aren't

Counting free storage space on macOS has become somewhat uncertain after Apple made some basic changes in APFS (the Apple File System) a few years ago. It’s sort of because there’s more than one way to count free space, depending on how purgeable files such as Time Machine local backups are counted. For example, on my Mac at this moment:

Photoshop Scratch Disks preferences says there’s 60.37 GB available

Apple Disk Utility says there’s 64.81 GB available

Apple macOS Finder says there’s 100.35 GB available

Apple About This Mac says there’s 101.75 GB available

A third-party utility says there’s 100.3 GB available 

(I need to take action because I really like to have well over 100GB free at all times)

My guess is that Photoshop is taking the more conservative count that’s more like what Apple Disk Utility is reporting, so it may say there’s not enough space sooner than if it was taking the less conservative count. You may have to free up even more space to convince Photoshop that there’s enough.

For more information on the confusing ways free storage is currently counted, see these articles by Mac expert Howard Oakley:

Free space on an APFS volume is an illusion

Where did all that free space go on my APFS disk?

The Finder confuses with wildly inaccurate figures for available space

From that, you can conclude that one possible quick solution is to go into Disk Utility and delete unneeded Time Machine local snapshots, if present. If those currently use a lot of free space, deleting them can cause the free space reading to go up significantly. However, do this only if you are sure your actual backup volume is up to date.