Pro Tools Ultimate Not Saving

[aftermid]

Hi,
This seems to be an intermitten problem with a session I am doing. About 20 min. into the session Pro Tools won't allow me to save and posts up an error message. I can't do a "Save As" either. I try creating a new session and importing session data from the problem session and then I get one of the two following errors: "Can't open because the session was created on a newer or older version of Pro Tools." which isn't the case.

Sometimes during the session I also get a "can't do a auto Session Back" error.
If I reboot and open the session, it saves fine, but I've lost my previous work. I now have to go to the session back ups folder and look for the newest session and import the latest audio files and reconstruct the tracks I did previous to the best of my memory.

The session is being done on a external 4TB LaCie drive with over 3TB of memory still available. I can't replicate this problem consistently. I've seen other posts on the forum with similar issues, but there from like 2008 or 2010, none give a real solution in the end.

As you know, Pro Tools tech support won't help without their ASC for a fee to help, unlike other softwares tech support departments that want to help their customers. I've actually used ASC for this problem before and they couldn't figure it out. They just gave suggestions for the problem that never really resolved. For the huge amount of money we pay for Pro Tools you'd think someone could answer a phone or email without charging, but I digress.

Don't know how to find out what it the cause of this. I am using a Mac Mid 2012 Pro 3.4 Ghz Quad Core with 12 gigs of ram.

Any suggestions or ideas would be greatly appreciated. Thanks in advance.