If you try to run a large, DB on an external HD, I would assume it would be slow via USB/Firewire. The only way I would consider it was via eSATA connection. eSATA is ~6 times faster than USB 2.0. I have an external, but I use it as a daily redundant sync back up.
I have no practical experience trying to run a DB on an external. My videos play OK, if I decide to watch any from my external, FWIW. But that is about as much work as I put it through.
I am hoping to build a new desktop this winter, with 3 separate internal drives, to go with the external.
*I agree, that datamining and old, observed hands, are way over rated.