Is the undetected drive different in any way to the one that is working? Is it formatted differently? I'd first suggest investigating for any differences between the drives.
Note that the below article appears to indicate GPT formatting is only supported by Symantec Encryption on Win8 UEFI:
http://www.symantec.com/docs/TECH149543
By the way, it's worth mentioning that you've posted in the Endpoint Encryption forums, that focusses on the former Guardian Edge product. You might get a better response by posting in the PGP WDE forums:
https://www-secure.symantec.com/connect/security/forums/pgp-wde-windows