The Photoshop scratch disk is your hard drive. Photoshop uses your hard drive as temporary “swap” space, or virtual memory, when your system does not have enough RAM to perform an operation. If you only have one hard drive or partition in your computer, then the scratch disk will be the drive where your operating system is installed (the C drive on a Windows system).
if you can open Photoshop, check to see what hard drive is set up as your Photoshop scratch disk. To do this, go to Photoshop > Preferences > Performance. Then look under Scratch Disks to see which one you are using and how much free space it has.
Option 1: Delete some files
If your scratch disk is very full (or the free space is low), try deleting some unwanted files from the drive (for me, “Macintosh HD” as seen in the image above) to make room for new ones.
Option 2: Delete temp files
Search your computer for temporary files. These files are typically named ~PST####.tmp on Windows and Temp#### on Macintosh, where #### is a series of numbers. These are safe to delete.
If photoshop doesn’t start and show Scratch Disk Full Errors
Luckily for you, the engineers have devised a shortcut to fix that: Hold down CTRL+ALT(PC) / CMD+OPTION(Mac) as soon as Photoshop starts to get this menu:
Select another disk with more room but do not forget the clean the files clogging up your hard drive.