Command+Option+Shift (Mac OS). Then, click Yes to the message, "Delete the Adobe Photoshop Settings file?"
I did this and was able to delete the settings file and then the window popped up again and no luck.
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Click the shortcut to launch Photoshop, then immediately press the Command and Option keys.
This will present you with a dialog where you can select a different drive with more space as the primary scratch disk.
If you only have one hard drive, you will need to clear up some space on it, for instance by moving files to an external drive.
Also check and make sure your hard drive is not actually running low on free space.
Go to the Apple menu, click on About This Mac, then click on Storage.
This worked for me I was getting the same message as above and command alt would not work for me. I deleted some large files and photoshopped opened up!