While Unlocking a Encrypted Drive.. It is possible for that drive to be unlocked for only the particular user and not all other users who use the system.
For example, there are two users...
User2, user1 unlocked a encrypted drive, now he locks the desktop and
user2 comes and open the system from his account. Now,
user2 also can access the drive which
User2 must not be able to open the drive. If he has the password the the drive then he might have access but not before that.