Selecting the best preferences to get the best performance out of your computer by selecting one or more hard drives for scratch operations. If your computer does not have enough RAM (random access memory) to perform an operation, Photoshop will use free space on any available drive, this is known as a Scratch Disk. You will find that Photoshop runs faster when you divide the Scratch Disk load. The Scratch operations are performed on a hard drive and will take place when Photoshop is using one of its vast selection of filters and adjustments. Photoshop detects and displays all available disks in the Performance preferences dialog box. This is where you can select the disks you want to use. You can assign additional hard drives here. This will speed up Photoshop's overall performance. Scratch Disk will not take place until you restart Photoshop.
Scratch Disks Options:
For Windows - Edit > Preferences > Performance
For Mac - Photoshop > Preferences > Performance
• Select the check box next to the scratch disk to chose what you want to use or clear the check box to remove what you do not want to use.
Photoshop holds scratch disk space as long as the application is open. To delete scratch disk space you must close Photoshop.