OPF clears and writes from MSX. I boot MSX-DOS from floppy. Write Nextor from it and press reset. If all goes well Nextor boots up and scans for an inserted USB flash drive.
I’ll put up a page with all the binaries to get going more quickly. Plus some instructions.
Btw. I realized that my design uses the SCC mapper. My Nextor image is adapted to handle that. If you prefer Ascii8/16 please let me know. Also good to have a switch that disables the rom during startup to prevent auto start. You cannot flash over the currently running image.
PS. If you have some pictures of your work I would love to see them 
I figured you are using the SCC mapper I do not mind using it. We can change to Ascii8/16 later. I'll consider my first version a 'testing' version.
First version of my PCB will look something like:
Why wuold you put sd card interface into this cartridge, when this is for usb interface? And in rookiedrive you can use usb sd card reader.
Well, you could save the USB port for a second drive, another function, etc without needing a hub. It won't hurt, for sure. 