I have an external USB hard drive that I use to backup all my data and designs. I also used to use USB flash drives fairly frequently until I got all my wiring ran for my network. When you say long term storage, are you meaning accessible at all times?
If so, an external hard drive has about the same life expectancy as an internal hard drive from my experience, even though the external may not be used as much (had to replace the drives inside the case two times on two different external cases out of the four external hd's I own). My dad constantly uses three usb external hd's as regular storage drives and hasn't had much issues yet, although he's still within about the first year/year and a half on most of the drives.