After encrypting an external drive, when you restart the computer with the external drive connected, you are prompted to authenticate with the external drive. After authenticating, you see the message: You need to format the disk in drive X: before you can use it.
You need to format the disk in drive X: before you can use it.
Check for upper and lower filter drivers on the system found here in the registry to see what is attached:
Symantec Encryption Desktop 10.x