I have a Samsung T1 Portable SSD

It was a little under $600. Stupid fast, but I havent tried to plug it in to any of my android devices yet.

I don't know how you're planning on delivering the drive, but an SSD is much more durable as it has no moving parts. If you're planning on using a mailing service which handles packages unkindly, I would definitely recommend SDDs.

Of course, external SSDs are expensive compared to HDDs and if you are looking for something in the 500GB - 1TB range it's going to cost you an upwards of $500 for one. Some things to consider: Is that worth it for the project? Do you really need that much space?

If you are also looking for maximum performance, USB 3.0 is faster and more universal than Firewire 800, and I don't think there are any SSDs out there with Firewire.

The Samsung T1 is the most popular external SSD on Amazon, but I've never used it so I can't vouch for it.