A scratch disk is a hard drive or solid-state drive that’s dedicated to storing temporary user data. It has no backup (intentionally) as it’s merely for temporary files, which helps create a virtual (cache) memory so things run a little quicker and more reliably. Things like your editing history within Photoshop are stored there but check here for more info.
So, when we see an error message that the scratch disk is full, there has been an excessive build-up of such files.
Step One: Clearing Cache
First and foremost, you can try clearing the cache. Photoshop has a built-in function for this, so simply boot up the application and head to the Edit menu, Then, head over to the Purge tab and press All. If it’s greyed out then they have already been deleted. You cannot undo this, so make sure you don’t need any previous versions of photoshop projects.
Step Two: Clearing Cache using 3rd Parties
Alternative to step one, you can use 3rd party software to clear the cache. This may be faster, but it’s always worth double-checking on the application itself. Third-party software can also clear cache from all around the Mac, which can create a lot of new storage space.
Step Three: Delete Temp Files
The next thing to do is to delete the temp files manually. Photoshop has many of these, so search for files ending with “.tmp” and begin with “pst”. There are forums explaining different ways of finding temp files, but you can literally search for them in the search bar.
Step Four: Clear Disk Space
The fourth step is to clear your disk space. Head to About This Mac and then Storage. Here, you can see how much storage is left on your startup disk. By pressing Manage on the right, you can take different actions to create more space. Emptying Trash, for example, will permanently delete the temporary files you just tried to delete.
Step Five: Check Activity Monitor
Checking the Activity Monitor can be very helpful. Here, you will see what is using up your RAM. Open your Activity Monitor by Heading to Finder, then Applications, then Utilities. Sort by the most demanding processes and begin quitting the ones that aren’t necessary. This can free up some RAM and CPU resources very directly.
Step Six: Reset Photoshop
You can always reset Photoshop to get things back in order. Sometimes files can become corrupt, and the best thing to do is start from scratch. Again, make sure you’re not relying on old project version backups so that nothing important is lost when resetting. Of course, you could always reset your MacOS too, but this may be overkill.
Step Seven: Change Your Scratch Disc
If you have more than one drive, you can always store your Photoshop temporary files on a different scratch disk. To do this, head to the Photoshop menu, press Preferences, and then Scratch disk. From here, choose a different scratch disk - one that has more space so it doesn’t get full up. Alternatively, you can upgrade to an SSD that has more storage than your current one.