Here is a quick tip we got from photographer and soundman extraordinaire Matthew Monroe. If Photoshop on your Mac is running slow, or worse, giving you the “scratch disks are full” message, there are things you can do.
Let’s talk about scratch disks for a second: scratch disk is where Photoshop stores information which is too big to store in RAM. This can happen when working with big files, applying complex filters and other instances where your RAM is just not big enough. If you have not made any changes to your default Photoshop installation, there is a good chance that your scratch disk is also your system disc (which is also your documents disc).
The smaller the scratch disk is, the slower Photoshop will run. And if it’s small enough, you will get the infamous “scratch disks are full” error.
Luckily the solution is easy, or at least the immediate front line support. Just free some space on your disk. For Matt, it was the downloads folder which was hogging over 12 GB of disc space.