You chose a scratch disk, which is no longer available, or there is some problem with your additional disks (like duplicates in the /volumes directory). This can happen sometimes with network or removable volumes (external disks).
The code signing message is a known problem in 10.8, we're working with Apple on it.
(we had everything signed, but somehow 10.8 doesn't like some of them where 10.7 did)
Thanks, Chris. I did find a text file in /Volumes that didn't belong there - it was from an unrelated programming project of mine.
[[ admin removed personal contact info -- be careful what you put in your replies ]]
Well, despite Chris's reply, the situation persists. If I reorder two scratch disks so that the boot disk isn't first, I can't open PS w/o the errors. All disks are INTERNAL in my Mac Pro.
The permissions verification (and repair) functions of Disk Utility will not tell you whether a permissions problem is preventing Photoshop from using your non-boot scratch disk. That only checks to see whether directories and files have permissions which match the permissions that they were given at the time of creation or modification by Apple software installer packages.
My primary "scratch disk" is a non-boot volume. Here are the volume's permissions as seen at the bottom of a Finder Get Info window.
OK, thanks. Checking the Ignore Ownership box on the desired scratch disk fixed my problem.
[[ admin removed your personal contact info, again ]]