When your computer doesn’t have enough RAM to perform an operation, Photoshop uses free space on any available drive as a replacement, known as a Scratch Disk. Photoshop runs faster when you divide the Scratch Disk workload.

Let’s take a look at how to change your scratch disk settings, and what settings work well for you.

13. Edit > Preferences > Scratch Disks…

14. To enable or disable a scratch disk, select or deselect the Active check box. To change the scratch disk order, click the arrow buttons.

15. What disks should I enable?

Good question. Adobe have given the following information in order to help you decide…

For best performance, connect the scratch disks to a compatible port that has the highest bandwidth limit of all the available ports. The bandwidth limits for various ports are as follows:
Thunderbolt = 10GB/sec
eSATA = 600MB/sec
PCIe = 500MB/sec
USB3 = 400MB/sec
USB2 = 35MB/sec

To improve performance, set the scratch disk to a defragmented hard disk that has plenty of unused space and fast read/write speeds. If you have more than one hard drive, you can specify additional scratch disks. Photoshop supports up to 64 exabytes of scratch disk space on up to four volumes. (An exabyte equals 1 billion GB.)

If your startup disk is a hard disk, as opposed to a solid-state disk (SSD), try using a different hard disk for your primary scratch disk. An SSD, on the other hand, performs well as both the primary startup and scratch disk. In fact, using an SSD is probably better than using a separate hard disk as your primary scratch disk.

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 your operating system uses for virtual memory.

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

Defragment drives with scratch disks regularly.

From our experience, SSD (Solid State Drives) give awesome results. Not only as scratch disks, but also as install drives.