So now I'm not sure whether or not I should return this drive? The fact it had the errors initially makes me nervous - this drive only has about 6 months left on the warranty, so it it fell over again -more permanently- in 7 months I'd be kicking myself.
I don't have an opinion on what you should do now. Maybe Kano is right and you'd get the same deal from another customer like you, or maybe not.
But whatever you decide to do, remember that the manufacturer's warranty is just a guarantee made by the seller/manufacturer, which in no way replaces any other rights you already have. I believe in the EU there's a 2 year warranty period by law, but I ignore the details. In the UK the Sale of Goods Act allows consumers to ask for total or partial refunds if the product bought is not of satisfactory quality. And durability is one of the defining factors of quality. Two plus two is four and I obtained a free repair on a laptop whose motherboard got fried 18 months after purchase (and 6 months after the warranty's expiry date).