I know of one, although it is a little pricey, and the power comes from Firewire interface.
Lacie drives are very high quality, i have one and love it. I don’t know if this drive can do the trick of using 2 USB ports to get enough power, as some lacie drives do. However, they do have excellent docs so perhaps you could find something there
Buy a 1GB 2.5″ laptop drive and purchase a 2.5″ USB drive enclosure. It will likely not apply, but who knows with certain vendors, you should check if the enclosure has a hard drive capacity limitation.
If you are referring to the external hard disk drive which has the capacity of 1 Terabyte or more, I would answer that it is possible to buy or use 1 terabyte external hard disk drive. However, it is still better to purchase a lower capacity hard disk drive if you believe that you cannot use those spaces at its maximum.