DISK ALLOCATION ISSUE
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) Repair Disk Permissions with the Disk Utility.
5) Drive is full or corrupted.
6) Date and Time set incorrectly.
7) 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.