Occasionally, Pro Tools will give you the following error message: “The drive cannot be designated as an Audio playback/recording volume because this is not a valid audio volume in Pro Tools on OS X.”
This can happen for different reasons, and the fix is relatively easy.
1. Check hard drives in the Workspace (the Workspace is located under the Window tab) and make sure the (A) column is set to (R) and not (T)
2. Trash the volume.dbb file on all mounted hard drives.
3. Trash Pro Tools preferences at USERS/username/library/preferences/Pro Tools preferences.
This should get your system back up and running.