No, you can’t use them now. The magnets have degraded too much…? Of course you can use them!
If you don’t have any use for them, then please consider donating them to some organization like Wikimedia Foundation.
Send them back? How can you? You posted this question in 2015, but you bought the drives in 2010. What store has a 5 year return policy? I would love to buy components from them. I once bought an Intel quad core CPU that I didn’t use for years. It was sealed, boxed version with the cooler. I ended up building a new PC with it for myself.
WD Red? Where did you get that idea? Those are WD RE4, enterprise level drives. Raid edition! Not Red! Those existed before Red and the other color branding of Caviar drives.