It is sometimes necessary to remove a drive from a bootable system and slave it to another system. If using the WDE Admin Passphrase user, doing so can remove the WDE Admin Passphrase user from the disk after entering the passphrase.
This was a known issue that no longer occurs with Symantec Drive Encryption 10.3.1 MP1 (Build 13266). Once using this version, the WDE Admin users will no longer be removed for external drives.
If updating to the latest client is not possible, the workaround to this issue is to slave the drive into a system that is not managed by Symantec Encryption Management server and does not incorporate the WDE Administrator Passphrase policy. This will keep the current WDE Admin users on the disk.