Constructive use for excess storage.

Been meaning to solve this dilemma for a while, but have come up with nothing. So I'm asking the community here if they can think of anything.

Due to various reasons, I'v got 5 SATA HDD's lying around worth ~2.5TB of capacity between them. All are fairly old (3yrs at minimum), though they still work just fine.
I have no use for all this storage. Iv already got an SSD for a boot drive, and two 1TB drives for my storage needs. So these drives have largely just been sitting in my rig, stealing my electricity and warming up the case.

What should I do with them?
One of them has currently got Ubuntu installed to it, though I'v only used it twice so its little more than a fallback if my SSD/Windows fails. Might as well just re-format the drive and install Windows to it.

Anyway, any idea's on how I can use all these drives?

 
I assume your two 1TB drives contain the same data so you've always got backups?
Well, you can never have too many backups. I've got four external 1TB drives, they all contain the same data. Call it overkill if you like, but I don't think so.