From Sue Chastain,
Your Guide to Graphics Software.
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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!

A. 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).

You can change the scratch disk location and add multiple scratch disks from Photoshop Preferences. 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.

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.

If you're getting an error message that the scratch disk is full, it 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.

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. See How to Defragment Your Hard Drive for Windows or Defragment and Cleanup Utilities for Macintosh.

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.