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 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.
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 Drive3 for Windows or Defragmenting Your Hard Drive4 for Macintosh.
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.