The permissions error is with the scratch file or volume, not with preferences. Resetting the preferences was just a way to force Photoshop into resetting the scratch preferences (which won't help if the boot disk can't be used).
Reinstalling Photoshop also won't help, since the disk permissions issue will remain. Reinstalling the OS after erasing the drive might help - but that's a lot of trouble.
You need to check the permissions at the root level of your disk, and /private (not normally visible in the Finder, you'll have to use the unix command line tools or GoToFolder).
It's also possible that you have duplicate volume names in /Volumes causing problems (again, you would need command line tools to clean that up).
My guess is that your upgrade changed the user id, and thus you don't have the permissions you think you do on the boot disk.
Chris,
Thank you for responding. I think you're onto something. I looked in disk utility to see if I could repair permissions on the scratch volume, as Lundberg suggested, however the option to do so was greyed out due to this message: "Not available because the selected disk is set to ignore ownership". So I went to the partition, pulled up the Get Info box and saw that the "Ignore Ownership on this Volume" box was not checked. I checked it to see what would happen, and although the option to repair permissions is still greyed out in disk utility, now Photoshop starts up perfectly, even with the problematic scratch partition set as the primary Photoshop Scratch Disk.
Is this the wrong way to get to the right answer?
Concerning your other suggestions, I was able to use Go To Folder to get to the /private folder, and I saw that it said I could only read. I was able to add my user account and give it Read/Write permission through the Get Info dialogue box (I fear Terminal unless I have explicit instructions). Is this what I want? I also looked into the /volumes folder and apparently there is nothing within it, unless the folders are hidden I suppose. I checked permissions of my main drive (startup disk, on which all my applications reside) and they claim that I have read/write privileges. However I'm only viewing this in the Get Info box; is that not the right place to see/alter it? Because this whole time the scratch partition also claimed that I have read/write permissions and that was obviously not true, as it only started working right once I checked "Ignore Ownership on this Volume". But it's incredibly confusing because I could save files to the scratch drive from Photoshop; I just couldn't choose it as a scratch drive.
The short question is: is it ok that I simply chose to ignore ownership on these disks, and that I messed with the /private folder, and/or do I need to do anything else?
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