That means the drive is not designated in Pro Tools to be an audio drive to record to. In Pro Tools, go to the Windows Menu, choose "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.

If setting drives properly in the Workspace browser does not help, check the format of the drive. For example, a hard drive formatted for Windows (NTFS or FAT32) can be read and mounted on a Mac, but ProTools cannot use those formats in Mac OSX. In that case an audio record drive must be formatted as Mac OS Exteneded (aka, HFS+).