I've used several drives on serveral computers and servers, external drives too...
I find old laptops (~1998) tend to fail if the laptop fast if the laptop is moved around even only slightly.
Server hard drives have all mainly been fine, had 3 that have failed, one had only just arrived and failed upon starting up.
Had some internal desktop pc drives die, the most severe being a western union with a loud clicking noise that would fail to power up, sent it off for data recovery and they changed loads of parts - they couldnt get it back to life either.
I've used several internal drives as external using a little USB converter, never had problems with them. Got a western union 1TB external drive and a seagate 750GB external drive, I usually just power them up to store junk on them I'm not needing right away or get the junk off, and both are a few years old and running fine, one was connected to a server to read some large database files for just over 2 weeks constantly.
Obviously internal drives are better suited, external are made small for easier carrying/shipment, there's no cooling mechanisms in the 2 external drives I have, compare that to a 2001 external tape drive I have - inside is an internal drive connected to a scsi panel on the back, there's LOADS of 'spare/wasted' room inside the case and there's a fan on constantly blowing air out. If you're looking for a decent external system that you're going to have on 24/7 or the like then I'd look for an old external tape drive or casing from the early 2000s on ebay that has a fan built in.
have a look at that! Yes the price is dear, but forget that, that's actually the same case as the case my 20GB tape drive is in but mine has a white/grey colour scheme and different backplate, something like that is perfect for an external hard drive and has a DECENT power supply built into it that you'll probably find is of a much better quality than the cheap chinese ones, there's probably a few ones without tape drives knocking around on ebay.
Edited by n_K, 06 January 2013 - 17:49.