So my burst test server has been running a few days so I connected to it to check everything was ok.... it wasn't.
The 250GB external drive I had put an extra plot onto had failed, no longer seen by the server.
I disconnected it and reconnected, the classic have you turned it on and off again, and all was fine. Looks like it is just a stability issue with the cheap enclosure and the constant HD Wakeup signals it was being sent. The enclosure was something I picked up on AliExpress ages ago for about $5 USD so to be expected I guess.
My take away from this is that if you want to run external drives with plot files, as you see many people do, then use a quality enclosure or you will spend ages trying to keep them running.
For now I will just stick with the original 500GB plot on the server drive as this is just to test mining and see what it is about.
Woz