I'm not sure about the life span of the physical drives, I have one that's about 5 years old and still going.
As stated already the external's use 2.5/3.5 inch internal drives, in cases, with a PATA/SATA to USB interface.
Personally I can't stand USB externals; the USB interface just kills it for me. But I would use externals with faster interfaces. As back-up devices they can be ok but try and shove a whole load of Gig's through the USB interface, and...... and....... and....... wait.
I know someone that uses a USB external drive to stream audio in real time to DAW application and wonders why he's getting pops/clicks/audio drop-outs when working with medium sized projects.