The tips outlined below came from this site.
First, if you can open Photoshop, check to see what hard drive is set up as your Photoshop scratch disk. To do this, go to Photoshop > Preferences > Performance. Then look under Scratch Disks to see which one you are using and how much free space it has. (FYI—I've included these steps for future visitors to this thread.)
Option 1: Delete some files
If your scratch disk is very full (or the free space is low), try deleting some unwanted files from the drive (for me, "Macintosh HD" as seen in the image above) to make room for new ones.
Option 2: Delete temp files
Search your computer for temporary files. These files are typically named ~PST####.tmp on Windows and Temp#### on Macintosh, where #### is a series of numbers. These are safe to delete.
Option 3: Defragment your Hard Disk
I hope one of these methods works for you!
Hello, welcome to the forum!
Since you cannot start Photoshop, you cannot change the scratch disk allocation in its preferences.
Luckily for you, the engineers have devised a shortcut to fix that: Hold down CTRL+ALT(PC) / CMD+OPTION(Mac) as soon as Photoshop starts to get this menu:
Select another disk with more room but do not forget the clean the files clogging up your hard drive, as described in the other messages.
Thank you! This is the first useful answer I've read and I've looked on every other website ever. It's like most people either can't read the problem or refuse to understand.
"I can't start Photoshop because it says my scratch disk is full. I have plenty of space on my drive."
"OK, open up photoshop..."
"First you want to start up photoshop..."
"After photoshop starts..."
"You want to go to the option menu in Photoshop..."
"Defrag your hard drive. I don't know why that would help but I heard about it in the 1990s."
"Have you tried turning it off and on again?"