Scratch Disk

Along with Dave's great information above, I want to add that you can add external drives as scratch disk space too. Add an external drive to your machine and in Photoshop>Preferences>Scratch Disks, check the new drive you just added. You will need to restart after doing this. So, if you can, save your file to a thumb drive or another backup drive before you do this. Then when you restart, you will have more Scratch disk space while you do the clean up of your hard drive as Dave suggested.

Michelle

Hi

Photoshop uses space on a disk to swap information between Random Access Memory and the hard disk. It does this to increase its working space and it is required no matter how much RAM you have installed. I have 64GB of RAM here and my scratch disk file is currently almost 12GB. That is no problem as my main scratch disk has 1.2 TB of free space on it.

Ideally you set the scratch disk to a spare disk, or at least one where there is lots of free space. You choose which disk(s) to use as scratch disks in Preferences > Scratch disks.

If you have only one disk and are running short of space then you need to clear out some free space on that disk. You can do this by working through your system, doing some housekeeping, and removing old temporary files etc. These can build up and use an incredible amount of space.  I can't advise the best way to do that on a Mac as I use Windows here, but it is all about clearing space.

Dave