I also would like this. I have a pocket sized PC. 8GB Ram, 256GB eMMC, 1TB NVMe, Quad Core 2+GHz CPU. I have a small 8GB Flash drive I use for Unraid, and have the NVMe for my storage drive. I would like to use the 256GB eMMC drive for storage of ISO files. ISOs take a LOT of space, but are rarely used, so having them on eMMC is not going to hurt my performance, and would be a better utilization of my available storage.
Unattached drives can't see the space due to lack of Kernel drivers for the eMMC. With my setup, I am not going to be having an issue with Licensing the number of drives I have in the system.