The nice thing about external USB drives is that you can position them some distance from your computer. Back before I switched to LTO tape, mine were taped down in a low/no traffic area. But, yes, they're fragile, almost certainly the most fragile backup media you can use, and my first set didn't survive this http://www.ancell-ent.com/1715_Rex_Ave_127B_Joplin/images/, at least not without reporting so many errors I gave up on them.

And you're of course right about the cloud, especially in the US; I've got rock solid AT&T ADSL, but it's very A(symmetric) and has a 150 GiB monthly cap, so only my very most important stuff gets saved to rsync.net, which e.g. saved my email from the tornado.

Hard Disks are not an archival format. In addition to the standard data retention issues... arms can rust and get stuck. There are simply more moving parts to a mechanical hard drive than either optical storage or tape storage.

Archival hard drives exist of course, but are at a cost disadvantage compared to tapes or optical media.

At least while we're waiting for bigger SSDs. Not sure what the expected lifetime of a (theoretical) 4TB SSD that's used mostly in append-only manner is. Will the drive outlast the USB3/SATA interface it comes with? It should at least be pretty resistant against physical wear-and-tear.

The storage cells are essentially capacitors, they store a bit of electrical charge. Eventually that'll leak away, unless the controller is sophisticated enough to do DRAM style refreshing every N years. Or you do it.

SSDs are way too young to depend upon, as far as I'm concerned. For that matter, if you really screw up and have to recover data from a drive, they're much more opaque than hard disks.