It may be a security feature built-in to whatever password-protection software you used for it on your office PC.
Stops it being moved to, and accessed on, a different computer (ie if the drive ever got stolen).
You might get away with just installing the password-protection software on your home PC.
Otherwise, you'll have to take the drive back to your office PC and remove the password protection using the same software you used to set the password in the first place.