That means the drive is not designated in Pro Tools to be an audio drive, this is a known, not uncommon, glitch that sometimes happens with external Firewire and USB audio drives. This does not mean there is a problem in your system, but likely just that a drive was accidentally disconnected while Pro Tools was open.

In Pro Tools, go to the Windows Menu, choose Show Workspace, and make sure under the A (for Audio) column the drive in question is labeled with an R (for Record). T (Transfer) and P (Playback) do not allow recording. Changes should take effect immediately and without restart, you can simply load or create your session immediately.