I have two Western Digital externals that I've been happy with. The USB interface means colossal transfers (10s or 100s of GB) are slow, but the speed is adequate for incremental backups, HD playback, DVD ISO burning, etc.
If you need more speed (eSATA), then building your own external might be cheaper than buying a premade one. The premades position eSATA as a pricey premium feature, despite the fact that it's almost free to add a eSATA interface to the SATA drive contained within.
Originally Posted by holmes4
It doesn't matter, really. I prefer the small drives and use a 320GB Western Digital model. The 1TB models are all using desktop drives at present and are larger than the Mini 9 in some cases.
Several manufacturers (Seagate and WD at least) make 2.5" form factor 1TB drives. They're taller than normal (12.5mm vs 9.5mm), so they won't fit in most laptops, but they'll also available as USB-powered externals, which will work with any computer.