Time Machine disk is not unlocked automatically

I have a FileVault2-encrypted Time Machine disk connected to my MBA through a Thunderbolt display in my office (the disk is a Buffalo Thunderbolt drive).

When I leave the office, I eject the drive. put my MBA to sleep and disconnect it from the Thunderbolt display (the drive still connected to the display).

When I return in the morning I reconnect the display, wake up the MBA and log in. But then I always have to manually unlock the external drive with Disk Utility. Otherwise it won't show up in the finder and Time Machine won't make any backups. That's annoying, because I often forget this procedure and TM silently fails to make backups.

Is there a way to automatically unlock the drive when I login in the morning, just like it does when I reboot the computer?

(the passcode for the encrypted drive is in my keychain)