Copy the contents of the ROM disk to your SD card, then erase the ROM disk in the recovery menu. Then it will boot straight from your (editable) SD card.
Select all ROM disk files with the space bar, then use Multi Mente’s copy command by pressing “C” and then typing “B:”, followed by the enter key. Copying should take maybe 10-20 seconds.
How to enter recovery mode is described in the manual on msxcartridgeshop.com. In short, press up key while booting, then use the F3 option. There’s also instructions how to restore the ROM disk if needed.