Intel Quark SoC X1000 Software User's Manual page 44

Troubleshooting tips:
To boot from SD/USB/eMMC, the grub instance embedded in the SPI flash is
hardcoded to search for a boot/grub/grub.conf file in partition 1 on the media
device. This is compatible with the factory formatting of most SD/USB devices. By
default, the UEFI firmware does not try to boot from SD or USB, it is handled by grub.
If you use an SD or USB device that has been reformatted after manufacturing, you
might experience problems booting from it. First, try to boot with a different memory
device and see if the problem goes away. If you isolate the problem to a specific SD
card, you can restore the factory formatting using this tool from the SD association:
https://www.sdcard.org/downloads/formatter_4/
It is not recommended to use normal operating system tools to format flash memory
devices.
Intel® Quark™ SoC X1000 Board Support Package (BSP)
Build and Software User Guide
44
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Document Number: 329687-011US
May 2017