Question: What is the Photoshop Scratch Disk? How Do You Fix "Scratch Disk Full" Errors?
Rosie writes: What is a scratch disk? And more importantly, how do I delete its contents because the program is not letting me use it anymore because apparently the 'scratch disk is full.' Please help, this is an urgent matter!
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).
Setting Up Scratch Disks
You can change the scratch disk location and add multiple scratch disks from Photoshop Preferences (File menu > Preferences > Performance). Many power users like to create a dedicated hard drive partition for the Photoshop scratch disk. Although Photoshop will function with a single scratch disk on the system partition, you can improve performance by setting the scratch disk to be the fastest drive in your system. Other useful guidelines for setting scratch disks are to avoid using the same drive where your operating system is installed, avoid using a drive where the files you edit are stored, and don't use network or removable drives for a scratch disk.
Note: If your computer has a fast solid state disk drive (SSD), you should use the SSD as your scratch disk, even if it is your system drive.
Delete Photoshop Temp Files
If Photoshop is shut down improperly or crashes in the middle of an editing session, this can leave fairly large temporary files behind on your scratch disk. Photoshop's temp files are typically named ~PST####.tmp on Windows and Temp#### on Macintosh, where #### is a series of numbers. These are safe to delete.
Clear Disk Space
If you're getting an error message that the scratch disk is full, it usually means you need to clear some space on whatever drive is defined as the scratch disk in Photoshop Preferences, or add additional drives for Photoshop to use as scratch space.
Defragment Your Hard Disk
It is also possible to get the "scratch disk is full" error, even if the scratch disk drive has free space. This is because Photoshop requires contiguous, unfragmented free space on the scratch disk drive. If you are getting the "scratch disk is full" error message and your scratch disk drive does show a good amount of free space, you may need to run a disk defragmentation utility. For more information, see How to Defragment Your Hard Drive
for Windows or Defragmenting Your Hard Drive
Scratch Disk Errors When Cropping
If you are getting a "scratch disk full" error when attempting to crop an image, it's likely that you inadvertently have size and resolution values entered in the options bar for the crop tool, or you entered values in the wrong units. For instance, entering dimensions of 1200 x 1600 when your units are set to inches instead of pixels is going to create a huge file that could trigger the scratch disk full message. The solution is to press clear in the options bar after selecting the crop tool but before dragging a crop selection. (See Fixing Problems with Photoshop's Crop Tool
More on Scratch Disk
For more on how Photoshop uses RAM and scratch disk space, see 'Memory allocation and usage (Photoshop CS)'
from Adobe, or look up "assigning scratch disks" in the online help for your version of Photoshop.