Padlock Symbol on Drive Icon?

[bbkim]

Has anyone seen a disk drive icon with a small padlock symbol at the left bottom corner? I've never seen this before. I have two drives show up on the desktop with padlock symbols. I can only read from their top directory. But I can read and write to their sub folders. One is my Time Machine backup drive and the other is not. I checked their permissions on the "Get Info" windows and found nothing out of ordinary.

What does this padlock means and how can I get rid of this "padlock"?

Any help will be appreciated.