It's a moot question. Your disk is full, period. You need to clear out space ASAP.
Raster image editing moves large amounts of data, much more than any RAM you may have installed. So temporary working data are written to disk. This is the scratch disk. It's simply free disk space, and you need a lot of it for Photoshop to operate. Photoshop scratch files will quickly exceed 20-40 GB, and can sometimes grow to 500 GB or more.
Personally, I would never attempt to work with anything less than 500 GB free space for scratch.