Scratch disk reporting full with 200gb free space

If you go into Photoshop Preferences / Scratch Disks and it reports less free space than macOS does out in the Finder Desktop, this sort of thing has been seen on recent versions of macOS. It may be related to changes Apple made about how macOS counts free space.

The short answer is:

Free up more space until Photoshop thinks there’s enough free space for the size of document you’re trying to open. It won’t help to look at the free space numbers, because of the explanation in the “geeky answer” below. Just keep freeing up space until Photoshop decides there’s enough. If it isn’t practical to free up so much space on your startup volume, do the next thing below… 

Sometimes I attach a large, empty, fast external SSD (for example, 1TB) and assign it as the primary scratch disk. If you do this, make sure the SSD has at least 500GB free, the more the better if you’re opening huge files. This part is nothing new. Over 10 years ago people were already saying to keep over 200GB of free space on the scratch disk if you want to open very large files, and files are even larger today.

Geeky answer:

On current Macs, very different amounts of free space can be reported depending on where you look (the macOS Finder, Apple Disk Utility, various non-Apple applications…). Right now on my Mac, Photoshop says there’s 216GB free, Apple macOS Finder says there’s 265GB free, and Apple Disk Utility says I have 234GB free. So even two apps by Apple are reporting different free space numbers. Obviously, that is weird. I don’t have the technical background to know whether Photoshop, Finder, or Disk Utility is the most correct about available free space. And there is actually a possibility that we normal users can’t ever know exactly which amount is the right one, because the way macOS reports free space today is extremely confusing. A couple of articles by the excellent Howard Oakley help explain how this came to be:

The Finder confuses with wildly inaccurate figures for available space

Where does macOS get its volume free space figures from?

Based on the end of the first article, one thing you can try to free up more space right now is to go into Disk Utility and select the startup volume. If there are any local Time Machine snapshots listed for that volume, delete them and see if Photoshop thinks there is enough space after that. It’s best to delete them only after making sure your backups are up to date, because if they are, you shouldn‘t need the local snapshots any more.

These might help --

Troubleshoot scratch disk full errors in Photoshop

Known issues in Photoshop