See ROCK Storage Basics ... there are sections on internal and external drives pros/cons and how to.

I don't use ROCK myself, but did set one up for a friend once, iirc I just followed Roon's help guide.

As far as I know it is possible to have both a internal and external music folder drives ... each one would be on it's own share under \\ROCK\Data\Storage ... but I don't recall what the naming convention is sorry.

861v6+ID40, HD621, 8000.2, 7200HC, 7200.2, SKYQ Silver, PS3, PDP-LX6090, QNAP TS870-Pro (8*6TB), Roon, iPadAir2.
[In Reserve] C15, MD600 (2*3TB), TwinStore RM (2*2TB)