Mac storage basics · Caches, logs and temporary files
How do I stop Photoshop filling my startup disk with scratch files?
Short answer: In Photoshop → Settings → Scratch Disks, choose a fast external drive instead of the startup disk and restart Photoshop. Scratch files are removed when it quits.
Photoshop uses free disk space as temporary memory (the scratch disk). In Photoshop → Settings → Scratch Disks, tick a fast external drive and untick your startup disk if it's short of space. Restart Photoshop to apply.
Scratch files are normally removed when Photoshop quits; leftovers after a crash disappear when you restart the Mac. A "scratch disks are full" error usually means the startup disk itself is nearly full.