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.
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.
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.
If your photoshop is not opening you can use the shortcut to fix that: Hold down CTRL+ALT(PC) / CMD+OPTION(Mac) as soon as Photoshop starts you will get a menu to select another disk with more room.