Things To Consider
Size/Form Factor – How much storage space do you need? How big of a drive, physically speaking, do you want? If it will be on your desk, a 7200RPM, 3.5" drive will be best. These can hold up to 2TB on a single drive. Do you want RAID1 redundancy (two of the same drives mirrored in one enclosure), so that if one drive fails you still have a backup? If you need to have a portable solution, get a 2.5" drive that is lightweight and can be powered by a single USB outlet.
Price – Get an external drive that fits in your budget! Again, think about how much capacity you will need in the future. 3.5" external drives are cheaper and larger than 2.5" portable drives. Remember that prices are always coming down as well.
Interface – Obviously, you want to have a USB 2.0 interface at the least. This is the most versatile and readily available method of connecting the drive to your computer, be it a desktop PC, notebook laptop, netbook, or Mac. Read my other guide to see the difference in transfer rates of the USB connection. Other interfaces to consider that are good to have are eSATA, FireWire (IEEE1394), and RJ45 (networkability, network attached storage, NAS). An eSATA connection would be ideal for video editing. The connection it provides is like having the external drive connected to the inside of your computer, it’s that fast. Firewire is better than USB, but nowhere near the speeds of eSATA. An RJ45 network connection is handy for using the drive across your network, but this is probably not as practical for video editing as it is storing large amounts of files as a server.
Build Quality – Do your research on the drives that you are considering. Find out what other people like and dislike about their drives. Do some find it noisy, hot, or difficult to use? Keep these things in mind because if it bothers a majority of the users, it is bound to bug you too!