cp /usr/lib/udev/rules.d/95-udevil-mount.rules /storage/.config/udev.rules.d
Clone the udev rules file ^
# check for blockdevices, /dev/sd*, /dev/sr*, /dev/mmc*, and /dev/nvme*
SUBSYSTEM!="block", KERNEL!="sd*|sr*|mmc*|nvme*", GOTO="exit" <= edit this
# check for blockdevices, /dev/sd*, /dev/sr*, /dev/mmc*, and /dev/nvme*
SUBSYSTEM!="block", KERNEL!="sd*|sr*|nvme*", GOTO="exit" <= to this, i.e. drop mmc*
Then edit the file using nano and change this block towards the top of the file ^ then save/reboot to effect the change.
The change means for any mmc* blockdevice it skips straight to exit and doesn't process partition mounting. NB: This will also prevent SD cards from ever mounting as those are also mmc* devices. I doubt you need that though.