6 GB is nothing. Your disk is full, period. You actually have a much bigger problem than Photoshop - your whole machine may freeze up at any moment. The operating system needs free disk space for routine maintenance, and it doesn't have that now.
Yes, move everything possible off your system drive. Store it on an external drive, USB thumb drives, whatever. Just get it off your system drive. When you've done that, run disk cleanup in Windows. Check as many options as you can for a thorough cleanup.
Don't stop until you have at least 50-100 GB free space, but preferably more. My standard advice is 250-500 GB for Photoshop to work efficiently.
The scratch disk is not an abstract setting, it's real, physical disk space. Image editing requires massive amounts of memory, much more than any RAM you may have installed. So Photoshop writes temporary working data to disk, aka the scratch disk.
If you have to work with limited disk space, reduce history states, if necessary all the way down to 1 or 2. Much of the disk space is occupied by history states, and this will significantly reduce the scratch file size.
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