If you are looking for an external hard drive just for backup purposes and you've got a decent Internet connection at home/work etc then you might want to consider backing up to the cloud instead (e.g. Dropbox, Box, Amazon S3 etc.) It's much cheaper, more secure and avoids the false feeling of security that using an external USB drive gives like Chris points out. The only inconvenience is that you need an Internet connection to backup/restore, but if it's for stuff you won't be accessing often then this usually isn't a problem.