This can happen for different reasons.
1) Check hard drives in the workspace and make sure the ( A ) column is set to ( R ) and not ( T )
2) Trash volume.dbb file on all hard drives.
3) Trash Pro Tools preferences. USERS/username/library/preferences/Pro Tools preferences
4) If you have run the "restore" or "reinstall" of OSX and choose the save applications and files option, Pro Tools will not function correctly upon being restored, one symptom would be the error as noted in the subject "The drive cannot be designated as an Audio playback/recording volume because this is not a valid audio volume"
Solution is to run the uninstaller from the Pro Tools 6x CD and reinstall.
Pro Tools at that point reacquires the drives as record volumes.