I have an iMac (8GB RAM/256GB SSD, macOS 10.12 fully updated) - with 216GB of free disk space.
There are no other disks installed on this system.
Recently, I get the scratch disks are full message when trying to use any tool in photoshop. I have purged all the scratch disks using the built in menus, validated that it is using the correct "Macintosh HD" disk in the Preferences, and verified that "Macintosh HD" has 216GB of free space.
I have run the Mac Disk Utility's First Aid on the partition, with no errors found.
I have restarted the computer, and photoshop several times.
I have started Photoshop Holding Command+Option and verified the MacHD is selected as the scratch disk.
Other programs, such as Illustrator, have no issues.
I'm really baffled at whats going on here, any ideas?
Have you tried resetting Preferences? Hold down Cmd-Opt-Shift and Start Photoshop. "Yes" when asked to delete the settings file.
I did one better, and reinstalled Photoshop, which seems to have fixed the error.
That being said, your suggestion might have saved me quite a bit of time.
The Preferences files keeps everything as you left it when you exit Photoshop. Sometimes in writing to disk, there can be errors. So keep it in mind if unexpected behavior shows up and you know you didn't do anything out of the ordinary.
Otherwise, good to know it's fixed for you.