I created a PSD in Photoshop CS5.  It has many, many layers and saves at a size of 387 MB.  When open, it is 2.5 GB.  It opens fine in CS5, but when I try to open it in CS6 the progress bar that's seen when loading slows to a stop and stays there.  I tried opening smaller CS5 documents.  The largest document I could open in CS6 was a 297 MB file.  Anything larger would not load.

I have my main hard drive and another hard drive designated as scratch disks.

I have the optimize PSD and PSB files checked in preferences.

My hard drives are defragmented regularly.

Why can't Photoshop CS6 open anything larger than 297 MB?