So exactly how do you copy files from internal folder?
I open a network share to make them accessible and then use Windows Explorer or AlwaySync to copy them.
Revisiting this topic as in the mean time I have moved from a ROCK to a Nucleus+ & my library is increasing in size, and even though I have a QNAP NAS I can store music on, I’d like to try & keep it all on an internal SSD, so picked up a 4Tb SSD during Amazon Prime Day a few weeks back, but this is really making me wish that the @Roon would create a decent utility that would allow users to put a new ‘drive’ in to a caddy, plug that in to the Nucleus (Or ROCK) and migrate everything over from current internal drive to the new drive and then just open box & replace new drive! 
One can only wish…
It’s been asked for (but no guarantees that it will make it into Roon OS 2.0)
Thanks @Geoff_Coupe ! I’ve now added a “+1” on there