Pinebook Pro doesn't boot most of the time unless connected via USB-serial!
Yes, uboot (and idbloader) is your bootloader
I think mrfixit is bsp uboot (older), manjaro is mainline
No power flows thru serial connection
emmc is just a slightly different mmc, timing and bus width
uboot enumerates in order found often emmc is MMC 0
(since bootrom search order is SPI, emmc, SD, usb for program write)
Do be aware that with no uboot (blank emmc, no SD card) computer will appear dead
Also, with bad uboot, computer will appear dead, remove "bad" card,
insert good card, still appears dead, do "extra long press",, 20+seconds(pwr button)
To blank emmc, any boot will do, so emmc boot is OK
(1st 16M is outside of any partition area, so mounting is irrelevant)
Do all 4 things in one go, don't reboot until mbr is restored
That it sometimes works means that it is only a little bit wrong(for emmc)
more wrong for SD cards which is why it never finds them
The error msg about 'mmc block read error' ,, that is the problem
--edit--
I have gone thru a problem like this, but different with TV boxes
a different rockchip cpu, 3328 and non-standard wiring to regulators
uboot dtb was ok, both SD and emmc readable (boot order for this ,, SD, emmc, usb for rkdeveloptool)
the kernel would load, then kernel dtb and lose power to SD card, not find rootfs, cause card was dead
Needed different kernel dtb, as I type running manjaro with pinned 4.4 kernel (cause I don't have 5.x dtb)
So, the dtb (device tree blob) sets various hardware parameters, memory timing, regulators, all peripherals
kernel has one, so does uboot (simpler) internally
I think mrfixit is bsp uboot (older), manjaro is mainline
No power flows thru serial connection
emmc is just a slightly different mmc, timing and bus width
uboot enumerates in order found often emmc is MMC 0
(since bootrom search order is SPI, emmc, SD, usb for program write)
Do be aware that with no uboot (blank emmc, no SD card) computer will appear dead
Also, with bad uboot, computer will appear dead, remove "bad" card,
insert good card, still appears dead, do "extra long press",, 20+seconds(pwr button)
To blank emmc, any boot will do, so emmc boot is OK
(1st 16M is outside of any partition area, so mounting is irrelevant)
Do all 4 things in one go, don't reboot until mbr is restored
That it sometimes works means that it is only a little bit wrong(for emmc)
more wrong for SD cards which is why it never finds them
The error msg about 'mmc block read error' ,, that is the problem
--edit--
I have gone thru a problem like this, but different with TV boxes
a different rockchip cpu, 3328 and non-standard wiring to regulators
uboot dtb was ok, both SD and emmc readable (boot order for this ,, SD, emmc, usb for rkdeveloptool)
the kernel would load, then kernel dtb and lose power to SD card, not find rootfs, cause card was dead
Needed different kernel dtb, as I type running manjaro with pinned 4.4 kernel (cause I don't have 5.x dtb)
So, the dtb (device tree blob) sets various hardware parameters, memory timing, regulators, all peripherals
kernel has one, so does uboot (simpler) internally