I just upgraded to CS6 and updated a volume to Mountain Lion (latest version) — a disk that I was using for Scratch. Now, when I attempt to open Photoshop I get a "Could not open a scratch file because the disk is not available" message then closes.
I know Photoshop didn't have a problem with this particular volume prior to Mountain Lion. I made the Library file visible in ML in case PS needed something there as well as trashing the prefs & permission repairs but still no joy. Switching the Scratch volume to another (non ML) disk let's PS open no problem.
PS 13.01, Snow, MacPro 8gb RAM
Try repairing permissions on that volume. The upgrade probably changed the ownership of the volume (and the folders).