IIRC there is no support in Roon OS for such a configuration (you can’t format a second drive). Your report seems to confirm my memory. If you format the drive somehow in the right format it may show up (or not) alongside the other music drive – but this is tinkering and not supported AFAIK. If this doesn’t work or you don’t want to tinker with your ROCK then go back to a supported configuration with one drive for your music only. Another option is of course: Use a (supported by Roon Labs for Roon Server) OS of your choice that supports that hardware setup you have, install that and then Roon (Server) on top.
Please also note that the NUC13 series is not supported by/for ROCK so you’re on your own here.
Some threads regarding the NUC13 can be found using the forum search. One I add below: