Sometimes when trying to open a PT session that was previously saved on an external disk (LaCie 232GB), I get this dialog box:

The session is saved on a volume set to “Transfer” only.
If permitted, set the volume(s) in question to “P” (Playback)
or “R” (Record) in the Workspace.
Otherwise, choose OK to save a copy of this session
and its media to a valid playback volume.
The session copy will automatically open from the chosen location.
[Cancel] [OK]

Fix: Cancel out of the dialog, and open Window > Workspace. Look at the “A” column associated with the suspect volume. It probably shows up as “T” (for transfer). Make it “R” (for record).

(Amazing realization: this is exactly what the dialog box tells you to do!)