1. Run Photoshop
2. Go to Edit -> Preferences -> Performance
3. Click (mark) on your most empty and defragment Scratch Disk.
4. Use little arrows on the frame to move it on top of other disks ( if you have any)
It is recommended scratch disk to be other than the system disk where operation system is installed.
If you don't have other than system disk you have no options but to free up some space and split the disk in to two partitions. One for the scratch and other for the OS and all your programs and files.
If you have any other questions don't hesitate to ask.
Luck.
