Bit Locker: Relocking External Hard Drives

Question

I am running Windows 7 Ultimate with Bit Locker.  I have three external drives connected to the computer with Bit Locker installed.  This all works perfectly and I am easily able to unlock them and use the data....my question is, "how do I re-lock the external drive without shutting off the computer"?

There should be some simple way to re-lock the external hard drive so say if you leave your computer running, the hard drive is now locked again if you leave the room for awhile.  I can't believe this is not possible.  With True Crypt, it would only take a second to dismount the drive and it would be locked again.

Does anyone have an answer?

Thanks....Wayne