Photoshop supports up to 64 exabytes of scratch disk space on up to four volumes.

If you have more than one hard drive, you can specify additional scratch disks.

To improve performance, set the scratch disk to a defragmented hard disk that has plenty of unused space and fast read/write speeds.The bandwidth limits for various ports are as follows: For best performance, connect the scratch disks to a compatible port that has the highest bandwidth limit of all the available ports.If you make large changes to dense pixel layers (that is, using several filters on complex background images or many edits to large smart objects) you could need as many times the size of the original file as there are history states.Additional free space may be required based on the file type you're working with. It is recommended to have minimum 20 GB of free space on your OS hard drive while working with Photoshop. If you make small changes only, you need a minimum of 10 GB scratch disk space for Photoshop (with default preferences, brushes, patterns, etc).So, if the free space in your scratch disk is 10 GB, available space that Photoshop will consider in the scratch disk is 10 - 6 = 4 GB, which may/may not be enough for the current operation being performed. However, this is the minimum space that Photoshop tries to keep free in the scratch disk, and the available space that Photoshop considers in a scratch disk is the 6 GB subtracted from the current free space in the disk. Minimum free space on a scratch disk should be 6 GB for Photoshop desktop.

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