Had a problem today where I installed Photoshop 7, a really old version of Photoshop, and couldn’t open it. I had changed the scratch disk to an external drive and PS didn’t like that at all. The program from that point forward would not open, instead showing the error message: “Could not initialize Photoshop because the scratch disks are full.”
I finally found the solution here. Worked perfectly. While PS7 is opening, hold CTRL + ALT and you’ll have the option to change your scratch disks.