Thanks for the suggestion, but the PGD file resides on a RAID 1 array partition with no logged SMART faults. Also, other virtual drive PGD files are in the same partition and still functioning. If it's a hard fault, there is probably nothing that can be done, short of sending both physical drive units from the array to a data recovery specialist.
Are there any diagnostic tools to validate the shape of the file, whether it still contains properly structured encrypted data? What PGP Desktop UI tells about the problem is not very informative. It doesn't seem at all likely, that the entire 10GB volume is somehow damaged, as only a small part of it was written in the last session it was working. With the passphrase, theoretically it should be possible to recover anything not directly damaged or encrypted with a corrupted key etc.
Or am I incorrect in my understanding, that the actual AES 256 bit key is NOT written in the PGD file itself (which is the case with emails, where the data encryption/decryption symmetric key is written in the email, but encrypted itself with the PKC system key) and cannot be lost by any damage to the PGD file?
I did find a somewhat related report, but there were no replies and it was filed with WDE, which is not involved nor it was in that case.