Well, quite honestly since you haven't needed one of these devices YET, do you think you will be using it after your trip? Maybe yes but probably not. I would forgo spending as much as $240 on one of these and getting a few SD/CF cards because you will be able to use them later on or at least sell them and get some money back.
Thanks for your thoughts, Sean.
I haven't needed greater portable storage capacity yet because I haven't yet taken a long vacation with my new camera. The only time I'll ever use it: on vacations like this one, about once a year. I won't really use the either an HDD device or extra SD cards more than about once a year. I like your idea of selling the SD cards shortly after vacation to recover some of my cost. I could then buy newer, faster, higher capacity SD cards before my vacation next year, and then repeat every year's vacation. Buy, use, re-sell, upgrade next round. I'm just a little paranoid about <1> insufficient capacity, and <2> data loss.
Now that I do the math on capacity, I could probably get by just fine with SD cards. Picking up ~6 Sandisk Class 6 8GB SD cards for ~$240, plus the two I already have, at ~500 RAW images per card, would yield ~5,000 pics. Though, my composition still stinks, so I do take a lot more pics than necessary. But that's an awful lot of pictures! No way I'll take that many.
As for data loss, in some respects SD cards are safer - if one card goes bad, I only lose one card of data v. losing all the data on the HDD at once. Hmmmm...leaning towards more SD cards, now, and probably 4 more would do the trick. Gonna have to ponder this further...