That's a good question.
I have a Seagate external hard disk drive. It's Seagate 160 GB. I like it. I bought it 1 1/2 years ago. I am sure that there are many good external HD, increase HD's sizes, and less money by now.
Many companies do not tell the truth about their hard disk sizes. My HD has 160 GB, but it is really about 150 GB. Suppose, you bought a 300 GB, and it has to be about 270 GB. I am giving you a general idea.
Amazon.com: Seagate Technology 200 GB USB 2.0 and FireWire External Drive ( ST3200823A-RK ): Electronics
I recommend you not to buy an external hard disk drive on eBay because it is used and might wear out for less than one year. My suggestion is for you to compare them on:
NexTag - Compare Prices Before You Buy
mySimon - Price Comparison Shopping - Compare Prices - Unique Gift Ideas
PriceMac.com: Price comparison, Best Deal compare Macintosh prices
(I don't know what other website is better than these websites.)