Understand the frustration of the guy, as he probably just assumed it was the same drive without reading the second chance offer. You are partially at fault.
He is however being pretty obnoxious. At this point accept that he will give you negative feedback, because you shouldn't let him blackmail you.
What other accessories were there with the other drive?
Makes me wonder just how semi-literate nutjobs like that manage to turn a computer on, let alone use eBay.
He should have read the offer
What a noob. Forget about him and move on with your life.
Think i'd probably just offer to refund the guy £5 to shut him up. It'll probably end up costing you more than that when you refund his postage costs and pay out for postage again when you re-sell it.
If you want to sell on ebay again, send him £15 and move on with your life and forget it ever happened. Hopefully you will attract a more competent buyer next time. .