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Western Digital My Passport for Mac.
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My suggestion:

Buy this case:
http://oyendigital.com/hard-drives/store/U32-M.html

Then, keep an eye out for a 2.5" drive of your choice (either HDD or SSD).

Put it into the enclosure yourself (probably takes just a small screwdriver).

The advantage here is -- YOU built it, so you know what's inside. And if there's ever a problem with it, you can take it apart and check things out, since you were the one who put it together in the first place...
 
How much space do you need? How fast does it need to be? For not much over $100 you can get a 128GB SDHC card - if that's enough space for your needs, can't get much more convenient than that.
 
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