Hitachi has a line of external USB hard drives that are appearing now. They are fairly cheap, come in 1 and 2 TB versions, and they are large. (The size is not really a positive…) Anyway, they come with very nice drives inside (The 2TB version has a 7K3000 6Gbps Sata 3 with 64MB cache drive) which sells for more than the external version!
Anyway the problem with these drives is the USB cable! I have three of these drives plugged into an Apple Airport Extreme and all three of them disappeared at different times. After some testing I found that the USB cables seem to fail at the mini-USB end. This is strange to me since I plug these in and never move them, but in each case I was able to replace the USB cable and the drives all work fine now.
Hitachi, if you are listening, replace this cable! You are going to hurt your reputation over these drives and we can buy a much better quality cable for $0.86!
If you want to get a replacement, here is a source: