How to Fix the Photoshop “Scratch Disks Are Full” Error on Mac
"Could not initialize Photoshop because the scratch disks are full" stops Photoshop before it opens. "Could not complete your request because the scratch disks are full" stops it in the middle of an edit. Both mean the same thing: Photoshop ran out of memory for the document, tried to page the rest to disk, and the disk it was told to use has too little free space.
A scratch disk is not a special drive. On most Macs it is Macintosh HD, the same disk that holds your apps, photos and System Data, so the fix is partly inside Photoshop and partly about the rest of the disk. This guide covers both, in the order that gets Photoshop running again fastest.
What Photoshop Uses the Scratch Disk For
Photoshop keeps the open document, its layers, every history state and its caches in memory. When the share of RAM it is allowed to use fills, it moves the overflow to a temporary file on the scratch disk. Older history states go there first. A 16-bit or 32-bit file needs more of it, and so does a long session with dozens of history states on a large image.
Adobe's own advice is to keep at least 100 GB free on the primary scratch disk. That number surprises people with a 256 GB Mac, but it is what Adobe asks for, and the error appears long before the disk reads as completely full in Finder.
Get Photoshop Open Again
If the error appears at launch, Photoshop won't open until there is room, so free some space first (the section below lists where it usually goes), then restart Photoshop as Adobe suggests. If it still refuses with space free, reset its preferences as described further down, which lets it start with default settings.
Once Photoshop is open, go to Photoshop › Settings › Scratch Disks:
- Tick another drive besides Macintosh HD if you have one, and move the fastest drive to the top. Photoshop uses them in that order.
- The drive must be formatted APFS or Mac OS Extended (Journaled). Adobe lists only these two for macOS, so an external drive formatted exFAT for sharing with Windows does not appear here. Reformatting erases it, so copy its files off first.
- Restart Photoshop for the change to take effect.
A fast USB-C or Thunderbolt SSD works well as a scratch disk. A slow USB hard drive works, but every scratch write waits on it.
Make Photoshop Use Less of It
Free space is the fix; these keep the scratch file smaller in the meantime.
- Purge. Edit › Purge › All clears the undo buffer, clipboard and history. Adobe notes it cannot be undone, so finish the steps you might want to step back through first. Purge › Histories alone keeps the clipboard.
- Fewer history states. In Photoshop › Settings › Performance, under History & Cache, the default is 50 history states. Each is a snapshot of the document; 20 to 30 is enough for most work.
- More RAM for Photoshop. The same Performance pane sets how much memory Photoshop may use, 70% by default. Adobe advises against going above 85%. More memory means less overflow to disk.
- Watch Efficiency. Choose Efficiency from the status bar menu at the bottom of the document window. Below 95% means Photoshop is using the scratch disk; consistently below 90% means it needs more RAM than it has.
Free Up Space on Macintosh HD
This is the part that actually ends the error. Open System Settings › General › Storage to see what fills the disk, then work from the largest category down. Usual suspects on a Mac that runs Adobe apps:
| Where | What to do | Guide |
|---|---|---|
| Large files you no longer need | Find them by size and move them off or delete them | Finding large files |
| Lightroom Classic previews and cache | Lower the cache size, discard old 1:1 previews | Lightroom Classic cache |
| Premiere Pro and After Effects media cache | Clean it from the app's settings | Premiere Pro media cache |
| Old Creative Cloud apps and installers | Remove versions you no longer open | Creative Cloud leftovers |
| System Data | Caches, logs and local snapshots | What System Data is |
If the disk is close to full overall, a full startup disk covers the order to work in. Photoshop also gets slower well before the error, because macOS itself pages memory to the same disk: memory pressure explains what that looks like.
If the Error Comes Back with Plenty of Space
When the disk has well over 100 GB free and Photoshop still says it is full, the problem is usually the settings, not the disk:
- Reset the preferences. Quit Photoshop, then hold Shift, Option and Command while launching it and choose Yes to delete the settings file. Note your custom settings first; this returns everything to defaults.
- Check the scratch disk list. After resetting, set it again in Scratch Disks, as Adobe's steps end, and leave out any external drive that is not always attached.
- Reopen the file. Very large documents in 32-bit mode, or with hundreds of history states loaded from a long session, can outgrow any reasonable disk. Saving, quitting and reopening starts with an empty history.
Where Mole Fits
Mole's Clean lists Adobe's shared caches in your user Library, such as Camera Raw's cache, and holds them back while an Adobe app like Photoshop is running, so nothing is pulled from under an open document. Under System Data it lists Adobe logs older than seven days. It has no row for Photoshop's scratch file; that belongs to Photoshop while it runs and goes away when you quit. When the question is what else is filling Macintosh HD, Analyze shows the disk as a map of folders and files by size, so the largest item is obvious at a glance.
FAQ
Does the scratch disk need to be a separate drive?
No. Most Macs use Macintosh HD and work fine with enough free space. A second fast drive helps when you edit very large files every day.
Can I use an external SSD formatted exFAT?
Not on a Mac. Photoshop on macOS accepts APFS and Mac OS Extended (Journaled) scratch disks only. Reformat the drive, after copying its files off, if you want to use it.
Where is Photoshop's temporary file on Mac?
Adobe does not document the exact path, and it changes between versions. Quitting Photoshop normally removes it, so there is nothing to delete by hand.
Will more RAM fix it?
It helps, because Photoshop pages less to disk, but it does not replace free space. Photoshop still writes some data to the scratch disk on large files.