I own a couple USB hard drives, one of them is a Maxtor 500GB One Touch 4 (
this thing). It's a good drive, has been reliable for me so far, but I don't think is your best bet because after it sits idle for a while, it'll go on standby mode and next time you go to access it it'll take about 5 seconds to initialize.
If you have a decent bit of experience with computers- at least enough to know how not to handle a hard drive when it's out of a computer- you can save a good deal of money and get a much more reliable hard drive by buying a USB enclosure and (what would otherwise be) an internal hard drive. That's what I have connected to my machine now. It's ridiculously fast, is constantly ready for action, doesn't have any annoying crapware built into it, and if the enclosure ever conks out on me, I can stick it in my tower to rescue my data until I can find another good deal on an enclosure.