No devs?

I've done a bit of research and I think the issue is mmc_block isn't loaded during the initrd.gz

I have thus far been unable to figure out how to add it to that.

It is not easy to add a driver to the system, only the developers know how to do this, it is necessary to recompile the entire system.

Right. Then it's not going to work.

The pen drive boots via UEFI too and does it work? So the problem is not the drivers, not even the UEFI.

Yes. As I said. It works.

The problem may be in the way the first boot of Recalbox is done: when you burn the Recalbox image, it creates the BOOT partition and the RECALBOX partition, the first boot is created, the SHARE partition is created, with everything the remaining available space on the device. Maybe it is not able to format emmc to create the SHARE partition?

As I said, it can mount the EMMC once booted.

Did you say you can boot via SD card, or did I misunderstand (I do not speak English)? If you make a backup of the image of the pendrive after already having given the first boot, and burn that image to an SD card, does it work?

SD doesn't work.

I know that dual boot is not necessary, the link I went through was to help you better understand the boot process, so you have more information about where you can modify ... It is an example that can be useful, even if you do not do dual boot, I recommend reading.

I don't see much about the boot process in there. It's mostly just partitioning and setting grub.

Thanks for your help, but I think I'll need a dev to respond.

It boots fine from usb. When booted from usb, the emmc is recognised and mounts.
Dual booting isn't necessary.

I think the problem is that the emmc driver isn't loaded during the early ?bootloader? part of the process.

My knowledge doesn't extend far enough to know how to add that driver to that part.

As per title, I figure this little thing would be good for this.
However, while recalbox will recognise the emmc and mount it when booted from USB, when it's DD'd to the EMMC (even with the SD slot disabled), OR to an SD card (enabled), in verbose mode it stalls, repeating:

mount: mounting LABEL=RECALBOX on /new_root failed: No such file or directory
Waiting for the root device
(repeat)

As per the title, this little compute stick has NO Compatibility Support Module (traditional BIOS mode). It cannot boot in anything but UEFI.
It's got 1x USB port, and I'd rather not use a hub to use a controller.

Theory: the EMMC driver isn't loaded early enough during the boot process?

Does anyone have any suggestions short of "just use USB" or "use another machine"?

My linux knowledge is... enough to get around the command line with plenty of googling.

Thanks.