When attempting to encrypt external hard drive with PGP WDE feature, you unable to add the user key to the disk using "Add User Key..." button. When you select the user key to add in "Add user key" dialog and enter the passphrase for the external disk, the key is not added to the list of disk users. Same may happen in ADK enabled environment if you are trying to add key located on token.
Both keys have same Additional Decryption Key which already exist on disk drive.
This issue is fixed in the following release:
Symantec Encryption Desktop 10.3.2 MP1 (Build 15337) and above