I've been working on a psb file for a couple of days in CS6 and this morning the file would no longer open. I get a dialogue box that says "the file is not compatible with this version of Photoshop". I've been using the same version all along so I know this is not a version problem. My backup file has the same problem. I need help or I've lost a couple of days of work.
I can open a flatened version of the file in Illustrator CS6 but can't open a layered version. It would apear that the file has become corrupted in the layers somehow. Any thoughts on how I might be able to fix the original psb file and reserect the layers?
I have no idea if it is relevant to your issue, but have run into trouble with background saving after it was introduced with CS6, and a 6Gb file is going to take a while to save on the best of systems.
Can you tell us about the backup file? Did you save an earlier version with a different name, for instance? What are their file sizes?
Out of interest, I just knocked up a 6Gb file, saved as a PSB, and turned off the warning dialogue. (It was taking 12 seconds to load and save to a raid0 with two 2nd gen v'raptors) I then tried to force an error while saving, and the only way to make that happen would have been to shut Windows 7, and tell it I didn't care about lost data. I'm sure you wouldn't have done that!
So I guess your next step is to Google PSD file recovery options. A recent poster had success with one of them, and you might find the thread if you search.