I'd say it's best for a drive to be internally, but that's because it's harder to bump the drive while it's in use that way. I would say one of the main reasons external drives generally last shorter is because people treat them like books, or something.

Optimally, they should never be bumped or moved while the platters are spinning, which a lot of people do, either because they don't know, or from accidents. The same applies to hard disks in laptop computers, so these should be treated nicely when turned on too.