OP, I never "deployed" but I did work in Bosnia, Iraq, Afghanistan and Uzbekistan for many years. I made a lot of damned mistakes with external drives too. Every drive from cheap to expensive failed on me. One made it 1.5 years in Iraq and then failed in El Paso at the hotel on me.
Anyway, the drive that made it that long was a Maxtor drive with a SOLID aluminum case and decent cooling. What I would do is....
1. Stick with 3.5" drives and avoid the smaller lappy drives in an enclosure. I had 3 or 4 of them, everyone is defunct now.
2. Find a solid aluminum cased drive...maybe with a built in fan. (either way, you'll need to clean them out every now and then) Right now I have a Maxtor HDD and a WD (My Book) drive sitting right in front of me. I can squeeze the sides of the Maxtor and it does not budge. The WD is squishy.
3. After you find the case that works, buy the drive. If you find them both in one, you're good to go.
The reason I think the case is vital is for traveling. I assume you're going to be packing these in a suitcase or duffel as well. Either way, the way they strap those down so hard in the C130s - you need to have drive cases that can hold their own against pressure.
Also, have you thought about trimming 1 or 2 single purpose drives? That'll allow you to buy a 1 or 2 TB drive and backup the other drives to it incase of drive failure. If you get a private internet connection over there, let me know....maybe I'll setup an FTP server for everyone to put pr0n up for ya. 400 gigs really is not THAT much for a long deployment. lol (problem is when all your buddy's suck up my bandwidth...)