I use standard EIDE drives, whatever size I need, on "mobile drive racks"(Drawers), storing everything as simple data files. While I have all of my drives on drawers, you could just install one for your backup HD's. Cost about 20 bucks - includes one "rack", that installs in the PC case in about 20 minutes and one "drawer"(Which can be purchased seperately for about 15 bucks). Takes only a minute to pop drives in/out of their drawers and store "just the HD", however you like(Maybe in it's original static-free bag in a reasonably controlled temp/humid environment(A closet shelf/drawer in a bedroom? Or maybe rent space at a commercial data storage center?).
A "good-looking" brand is "Kingman"(? I think? Sorry, I'm in the wrong room to look - mostly metal as opposed to mostly plastic.). The one's at CompUSA(Under their own name) are fine. You can get them for SATA drives, too.
How long will they last? No idea. What will you play them on 15 years from now? No idea(Can say that about any format, but you'll "re-archive" as you must.). For now though, convenience and cost-effectiveness hard to beat...
A "tiny tip"? I've even thought of it myself -- but don't look at me! A friend who designed/built/maintained the machines for the world's largest maker of gold connectors, says that the gold connectors(Ends) on the average, thumbnail-size memory(Like used in digital cameras) cannot last long, at all(He says half a dozen in/outs can do them in...)! Just not enough gold. So, while their size increases weekly, their durability, likely, does not. Not an archiving option, at least for now. Matter of fact, as most of us have digital cameras, these days, his advice is to put one in the camera/memory device and leave it there, loading/offloading from the device itself.
TG