Photoshop not working due to "Scratch disk is full" even when I have 50GB disk space is available

It may be available on your system, but not available to Photoshop. Check under PS Preferences > Scratch disk, and make sure it's active and checked in the list.

External drives may not be usable if they're not formatted correctly. For instance, exFAT will not work under MacOS Catalina.

All that said, 50 GB isn't much and you should get more. For most of us, 500 GB is considered minimum.

When exactly do you get the message?

• when starting Phothop app from its icon (no image)?
• when starting a NEW image?
• when opening any file?
• when opening some files?
• when importing?
• when in the middle of editing?
• something else?