It depends on the drive, and thus it depends on how the OS handles the drive. If PS recognizes the drive, either USB or FireWire, you CAN use it, but I would recommend against it.
Since PS relies so heavily on the scratch volume, you want it as fast as possible and that's going to be local, internal. The minute you start running into an I/O backlog, I'm betting PS will croak. That's an opinion, not a fact.