Hi,
In addition to what Carl has written.
I found that when an external/USB HDD is in "sleep mode/energy saving" and media data is "drawn" from the HDD, PDR may display a "white out" non-responding screen (at some point). Using a free software such as "NoSleepHD" to ping the HDD, can ensure it doesn't energy save.
External drives can be used but if I had a preference, I'd stick with internal drives to edit from.
Just what I found.
Dafydd