1) Did you try to use a WDRT on bootguard to unlock the drive ?
Using Whole Disk Recovery Tokens
http://www.symantec.com/docs/TECH149068
2) I am right thinking that the drive which you hooked up in externally to another machine is not a "removable external drive" - ? if this could be a case an "encrypted removable external drive" would not work on other machine as the session key is always saved locally on PGPWDE01 on local machine. Otherwords your drive would not be visible on the other machine (which from the post I guess it is not the case)
While having the drive hooked up externally can you run the following commands please and post the feedback:
Windows x64
C:\Program Files (x86)\PGP Corporation\PGP Desktop>pgpwde --enum
Windows x32
C:\Program Files\PGP Corporation\PGP Desktop>pgpwde --enum
with
C:\Program Files (x86)\PGP Corporation\PGP Desktop>pgpwde --status
C:\Program Files (x86)\PGP Corporation\PGP Desktop>pgpwde --list-users
It can be that the disk is locked if the policy was setup to locked out the disk after X failure attempts on bootguard.
Try also the below command:
C:\Program Files (x86)\PGP Corporation\PGP Desktop\pgpwde.exe --unlock --disk 1 --wdrt "WDRT HERE" or --ap "admin passphrase here"