Photoshop uses your hard drive space as a “scratch disk”, a secondary memory resource to improve performance. If you have more than one hard drive installed in your computer, it is recommended that you allocate one of them as a scratch disk.
To set your scratch disk, choose Edit > Preferences > Performance (Mac: Photoshop > Preferences > Performance). In the Scratch Disk section of the panel, select the drive that you would like to use as a scratch disk.
For better performance, your scratch disk should be on a different drive than the one your operating system uses for virtual memory, and on a different drive than any large files you are editing.
Photoshop will use the drive listed first and then move to the next drive in the list if more memory is needed.
To change the order in which the drives will be used, select a drive and use the up or down arrows to move it up or down in the list. Be sure to place your system or startup drive on the bottom of the list.