ReadyNAS NV+ v2

Hmmm...  My ReadyNAS is sick

Hopefully i can get a steer from you good folks in the community.  I have an NV+ V2 with a mixture of 2,3 and 4 TB drives.  The unit is 4-5 years old.  Was running like a dream and then the following incidents occurred, basically in this order.

I started getting emails regarding sector errors on disk 4 several months ago, unperturbed i continued until i had the spare cash to buy a replacement drive.  Before this happened i started getting errors on disk 2.  By this point i switched the unit off until i have a replacement drive in hand.  I put a new 4TB into slot 4 and the unit started the re-sync.  As the re-sync was going o i was getting email regrading disk 2,  I kept my fingers crossed.  Sure enough the unit rebuilt Vol C with a lifesup! message.  i checked the logs and sure enough Vol c WAS REBUILT.  I was aware that disk 2 was about to fail so i switched the unit off until whilst i investigated what Lifesup! meant.  Not Good. I turned the unit on with a view to replacing drive 2 however now the unit refuses to mount Vol C.  When i go to the RAIDar panel all the lights are green and the system status is "up".

Now the questions I have are:

Do i bother spending the thicker part of $250 for a replacement drive for slot 2?

Do i contact premium support and buy a once only support product?

Do i go ahead and upgrade the firmware (1 release behind at the moment) in the hope this will kick the unit in the guts and get vol C mounted...?

Any direction the community can offer will be much appreciated.  At the moment I have nearly 7TB of movies I really don’t want to lose!