Photoshop scratch disk full on macOS Sequoia – fix

Quick answer

Purge Photoshop cache (Edit > Purge > All), then change scratch disk to an external SSD in Photoshop Preferences > Performance. If that doesn't work, free 15GB+ on your boot drive or reformat external drive to APFS.

Why this happens on Sequoia

I've seen this on a dozen Macs since Sequoia dropped. The culprit here is almost always Apple's new file permission restrictions in Sequoia — they block Photoshop from writing temp files to certain system directories it used to use. Combined with Apple's default 20GB reserve on the boot volume (which counts as 'unavailable' to Photoshop even if you have space), the scratch disk runs dry fast. Users see this after a macOS update or when they run Photoshop with 8-16GB RAM and a full SSD.

Fix steps (do these in order)

Purge Photoshop's memory caches
Go to Edit > Purge > All. This clears clipboard, history, and undo data. Photoshop will warn you — click OK. This recovers scratch space immediately if it's a memory leak issue.
Change scratch disk to an external drive
Open Photoshop > Preferences > Performance > Scratch Disks. Add an external SSD (preferably Thunderbolt or USB-C). Uncheck your boot drive. Restart Photoshop. The external drive must have at least 20GB free.
Free space on your boot volume
Sequoia reserves 20GB for system operations. Photoshop sees that as 'full'. Delete large files, empty Trash, use System Settings > General > Storage to clear caches. Target 30GB free minimum.
Reformat external drive to APFS
If you moved scratch to an external drive and still get the error, it's probably ExFAT or NTFS. Photoshop hates those for scratch. Backup the drive, reformat as APFS (GUID Partition Map) using Disk Utility. Re-set the scratch disk.
Reset Photoshop preferences
Hold Command+Option+Shift while starting Photoshop. Click Yes to delete preferences. This wipes your settings but fixes corrupted scratch disk configs. Re-apply your performance settings after.

If the main fix fails

Rarely, Sequoia's SIP (System Integrity Protection) blocks scratch access entirely. Try this:

Boot into Recovery (Intel: Command+R at startup, Apple Silicon: hold power button).
Open Terminal in Recovery.
Run csrutil enable --without fs — this disables filesystem SIP protections only.
Reboot. Photoshop should now access its temp folders. Re-enable SIP fully after fixing with csrutil enable in Recovery.

Don't bother with 'Repair Disk Permissions' in Disk Utility — that's a relic from macOS High Sierra and does nothing on APFS volumes.

Prevention tip

Keep 20% of your boot drive free at all times. Use an external Thunderbolt SSD (1TB minimum) dedicated to scratch. In Photoshop Preferences > Performance, set RAM usage to 70% max (80% on Sequoia causes pageouts). And purge caches weekly — it's muscle memory after you've done it a few times.