I ran with an external eSATA drive for a year, but it is not recommended. I think a significant concern would be risk of disconnect, or any number of other `performance issues.
How do you seprate the database and the actual video recordings. I have an ssd but no room in the case for a 3.5 drive until I build a new pc or buy one. I will have to use an external drive that I have laying around to have space for the clips