[aftermid]

Ok Session was stable for awhile, but now getting the same error message again:

Pro Tools was unable to Auto Backup your session.

If your session is on a read-only drive, you must disable "AutoSave" by unchecking "Enable Session File Auto Backup" (located in the Operations tab of the Pro Tools Preferences dialog) to avoid this error.

(Error was: Invalid argument (22) )

Also, when I click save at some point in the session, get error message that session can't be saved. So I have to close without saving and lose work. Totally frustrating. The drive in question has Read & Write permissions and obviously it does record tracks, until about 20 min into the session you try and save and you get the error. Saving before that usually is fine. Again, intermittent issue, I can't define why it randomly happens on this session.