Scratch disk full errors frequently happening when saving documents, trying to open from RAW and during random edits causing the program to shut down. The scratch disks have over 500 gigs available space. This is not a problem when I run PS 2018. Windows 10
If you have only one hard drive in your computer, you will need to find files to delete or move to another drive to make space. Otherwise, you can mount another drive, and specify the preferences to use it. Go to Preferences > Scratch Disks... and set the newly mounted drive as your primary scratch disk.
If Photoshop cannot launch because the currently selected scratch disk is full, hold down the Cmd + Option keys (Mac) or Ctrl + Alt keys (Windows) during launch to set a new scratch disk.