The Photoshop scratch disk is similar to virtual memory. For the best performance, you should set the scratch disk to a defragmented hard disk that has plenty of unused space and fast read/write speeds (rather than a network drive or removable media such as a Zip drive). Photoshop requires at least 1 GB of free hard-disk space, but more is recommended. If you have more than one hard disk volume, you should specify additional scratch disks.

Note: Adobe recommends that you set the primary scratch disk to a different hard disk than the one Windows uses for its virtual memory or paging file.

To set the scratch disk:

1. Choose Edit > Preferences > Performance.
2. Select the Active? box for each hard disk you want to contain a scratch disk.

Note: Unless you have a drive that has considerable space open, and is defragmented regularly, choose more than one drive, if one or more is available.

3. Click OK.
4. Restart Photoshop.