When your system does not have enough RAM to perform an operation, Photoshop uses a proprietary virtual memory technology, also called scratch disks. A scratch disk is any drive or drive partition with free memory. By default, Photoshop uses the hard drive on which the operating system is installed as the primary scratch disk.

Photoshop detects and displays all available internal disks in the Preferences panel. Using the Preferences panel, you can enable other scratch disks to be used when the primary disk is full. Your primary scratch disk should be your fastest hard disk; make sure it has plenty of defragmented space available.

The following guidelines can help you assign scratch disks:

• For best performance, scratch disks should be on a different drive than any large files you are editing.

• Scratch disks should be on a different drive than the one used for virtual memory.

• RAID disks/disk arrays are good choices for dedicated scratch disk volumes.

• Drives with scratch disks should be defragmented regularly.