For those of you who use Adobe Photoshop, chances are that at some point you will experience the “Scratch Disk Full” error.
What is a scratch disk?
and what is the solution to this problem?

The Adobe Photoshop scratch disk is your computer’s hard drive.
When your computer doesn’t have enough RAM to perform an operation, Photoshop will use the computer’s hard drive as a swap disk, or virtual memory to keep the program running.

The problem or error occurs when your computer’s hard drive is full.
So Photoshop has no more space to continue its operation.

Some of these methods you can try to do when you experience a scratch disk full error.

Setting the Scratch Disk Location

In the performance menu (Edit – Preference – Performance), you can change the location and add multiple scratch disks for Photoshop.

If you use Photoshop for editing large files, creating a hard drive partition specifically for scratch disks is a good idea.

A tip to try for scratch disk settings is to avoid using the same drive as your OS drive, and also avoid using the same drive as the save you are editing.

Notes: Solid State Disk (SSD) drive.
It is highly recommended to use this as a scratch disk if you have one, even if it is also your OS drive.
Because the SSD operation process is much better than the disk drive.

Clear Hard Drive Space

If this error appears, it is likely that the hard drive used as a scratch disk is full, aka scratch disk full.
So clean up the computer drive from junk files, delete those that are not important, increase the location of your scratch disk, or increase the Harddisk capacity.

Delete Temporary Photoshop files

Clean your drive from Photoshop temporary files.
If the Photoshop program crashes while you are editing, there is a good chance that there is a large size temporary Photoshop file left on your scratch disk.
Find and delete this file immediately.

On Windows OS, this temporary file is usually named ~PST####.tmp, where #### is a sequence of numbers. This file is safe to delete.

Defragment Hard Drive

Well, if after all the steps above you have done, but the full disk scratch error still appears.
The last step that can be tried is to defragment the hard drive.
This is because Photoshop requires a clean hard drive space, and is not fragmented on the scratch disk used.

More information about how Adobe Photoshop uses RAM and scratch disks is on the official adobe website.
Click Memory allocation and usage (Photoshop CS)

Source: bolosaholic.com