external hard drive that takes a beating...?

[pieter]

Laptop HD (2.5 and 1.8") tend to be a bit more robust than ones meant for desktop use.

Apart from that I don't think it matters that much what you get, since it sounds like you will only be using it when stationary, ie no vibrations when the drive is running.

If that is indeed the case there is no need for all kind of fancy gell casings, just put it someplace safe amongst eg clothing.

It is when being used (spinning) that the drives are most vulnerable, and laptops like Toughbooks, meant for use in harsh environments, have the drives in special shock absorbing cases to protect them.

Pieter