This page sums up the tasks referenced and suggested in the first post on reddit.com/r/SteamDeck Post date: 3 Mar 2022
It shifts focus over to simply be some kind of blunder with that specific SD card and the BIOS. You can see that
DiscoverUserArea can be viewed as here: https://bsdio.com/edk2/docs/ace/_sd_dxe_8c.html in the reference UEFI execution (composed by Intel BTW).
It’s not really a Linux thing since Linux has an alternate SDcard driver viewed as here: https://github.com/torvalds/linux/mass/ace/drivers/mmc/center/sd.c
So likely you simply will not have the option to boot from that SD card however Linux loads it fine whenever it’s booted. That makes sense of why you can utilize it fine in SteamOS despite the fact that you see that error.