If I buy HDs or DVDRWs (or other little things) from a different supplier than my main one, I write it on the part itself (usually on my warranty sticker).
Buy even if I'm not sure who it was bought from, a short phone call to the lab, I give them the device's S/N, and they tell me if I bought it from them, when, and how long the warranty is for.
Thought about attaching a barcode reader to my PC, and scanning it right into some DB, but haven't had the time to mess with it yet.