Photoshop 'Scratch Disk Full' when you have free space

You're in the middle of something—painting a mask, stacking layers, maybe running a big filter like Liquify or Camera Raw. Then Photoshop slams the brakes: 'Scratch Disk Full'. You check your C drive and see 200GB free. What gives? I've seen this exact thing on a client's machine last month—a graphic designer who swore he had plenty of room. He was right. But Photoshop didn't care.

Why does this happen?

Photoshop uses scratch disk space as virtual memory. When you run out of RAM—which happens fast with big files—it writes temp data to your scratch disk. The problem is usually one of two things:

1. The scratch disk partition is full. Even if your main drive has space, if Photoshop is pointed at a separate partition or an external drive that's nearly full, it'll choke. I've seen people set their scratch disk to a tiny recovery partition by accident.
2. The system drive itself is full of junk. Windows or macOS keeps a hidden temp folder that fills up with Photoshop cache files. That folder might be on a different volume than the one you see in File Explorer.
3. Permissions or drive errors. Sometimes the drive just can't write because of corruption or permission issues.

Don't waste time on generic 'clear your cache' advice—the real fix is targeting the right drive and cleaning the right folder.

The fix: step by step

Step 1: Check which drive is your scratch disk

Photoshop defaults to the boot drive, but it might have changed. Go to Edit > Preferences > Scratch Disks (Windows) or Photoshop > Preferences > Scratch Disks (Mac). Look at the list—there should be a checkmark next to the active drives. If you see a tiny partition or a USB drive that's almost full, that's your problem.

Fix: Uncheck that drive and check your fastest internal drive instead. Ideally, use your boot drive or a dedicated SSD. Click OK and restart Photoshop. Don't skip the restart—it won't take effect until you do.

Step 2: Free up space on the scratch drive

Even if the drive shows free space in Explorer, Photoshop might be looking at a specific temp directory. Here's what to do:

• Windows: Press Win+R, type %temp% and hit Enter. Delete everything in that folder. You might get a 'file in use' error—skip those. Then open C:\Users\[YourUsername]\AppData\Local\Temp and clear that too.
• Mac: Open Finder, press Cmd+Shift+G, type /tmp, and delete what you can. Also check ~/Library/Caches/com.adobe.Photoshop.

After clearing temp files, empty the Recycle Bin or Trash. Then restart Photoshop.

Step 3: Run a disk check

If the drive has bad sectors or file system errors, Photoshop can't write temp data even if there's free space. Run a check:

• Windows: Open Command Prompt as admin, type chkdsk C: /f (replace C: with your scratch disk letter). Say yes to schedule at next restart, then reboot.
• Mac: Open Disk Utility, select the drive, click First Aid, then Run.

Had a client whose MacBook had this exact issue—bad blocks on the SSD. First Aid fixed it, and Photoshop stopped complaining.

Step 4: Increase RAM allocation

Less RAM means more scratch disk writes. Go to Preferences > Performance. The slider shows how much RAM Photoshop can use. Push it to 70-80% if you have 16GB or more. Don't go over 85%—your OS needs breathing room. Apply and restart.

If it still fails

Check these three things:

• Antivirus or cloud sync software locking temp files. Temporarily disable OneDrive, Dropbox, or your AV. If the error goes away, add an exception for Photoshop's temp folder.
• Too many layers or undo states. In Preferences > Performance, lower the History States to 20 or 30. Each undo eats scratch space.
• Drive is dying. Run a SMART check. If your drive has reallocated sectors, replace it before you lose data. I've seen this twice in the past year.

That's it. Most people fix this by just changing which drive is the scratch disk or clearing temps. Don't overthink it.