sasagr I don't know if it's included in these builds but you can prevent u-boot from trying to boot from SD card/any external media by resetting uenv variables to default values. Run this command: fw_setenv upgrade_step 1 and reboot. If you will ever want to boot from SD card again, you will need to use toothpick/reset method.
I did it as you instructed
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# LibreELEC #
# LibreELEC – Just enough OS for KODI #
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LibreELEC (community): LE9-Kodi_18.2 (X8-X8H.arm)
LibreELEC:~ # fw_setenv upgrade_step 1
LibreELEC:~ # reboot
inserted the SD card (that was successfully mounted) before reboot cmd
and I got the message on TV screen:
Error in mount_flash: mount_part: Unknown file system
i need to remove the SD card again to have a successful boot from internal
Am I giving the wrong command or at a wrong level?