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It doesn't need to be 7200 rpm, just small and ideally super robust.

I have a Western Digital My Passport Essential 320 GB, it works great. Since it's a 2.5" hard drive (can usually be found in laptops), it's small, portable. No external power supply is required. I attached one to my wireless router as a NAS for sharing files, music, and backup. It's currently $80 on Amazon, click here to see.
 
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Exact same external as GamaFu's post above - it is a great little drive indeed. I have it hidden inside my desk and it works flawlessly as my "out of sight out of mind" time machine HDD.
 
Exact same external as GamaFu's post above - it is a great little drive indeed. I have it hidden inside my desk and it works flawlessly as my "out of sight out of mind" time machine HDD.

Same here; although my Passport is a 250GB. I intended to replace it with a desktop unit (3.5" drive) but realized that it was perfect in this role. BTW, TM has already rescued me at least twice. :)
 
I've got a Western Digital MyBook Home, connected by Firewire, and it works great. I think it's pretty nice looking, but it's no where near as small as the Passports.
 
If you want a small hard drive that works in Windows aswell,

get a Lacie hard drive. small and portable

Every external hard drive should work more or less the same, you should be able to buy any and it would work on PC, Linux, or Mac just fine. Some external hard drive comes with backup software though, depends on if you need it or not you might prefer one to another.
 
I love my Seagate FreeAgentGo 160GB external USB 2.0. It's extremely small and just works.

I also have MyBook Studio 320GB FW800 drive and it's top notch.
 
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