Maybe you don't understand?
The mbr is 1 (one) sector, often specific to THAT media (that it is on),, the very 1st sector
Uboot is at 8M, 2-3M big, idbloader (on SD/emmc) starts at 64 sectors (32K), about 1M big
When you copy (with dd) the 1st 16m, you also copy the SD mbr, it is likely not right
That is why you first of all,,save emmc mbr, and last, restore it
(or you could make note of sector numbers (eemc) and edit SD mbr to be correct FOR emmc, on emmc)
Be aware that uboot on SPI is different (location) than uboot on SD/emmc
The mbr is 1 (one) sector, often specific to THAT media (that it is on),, the very 1st sector
Uboot is at 8M, 2-3M big, idbloader (on SD/emmc) starts at 64 sectors (32K), about 1M big
When you copy (with dd) the 1st 16m, you also copy the SD mbr, it is likely not right
That is why you first of all,,save emmc mbr, and last, restore it
(or you could make note of sector numbers (eemc) and edit SD mbr to be correct FOR emmc, on emmc)
Be aware that uboot on SPI is different (location) than uboot on SD/emmc