When pin 20 clamped to GND, "SPI-FLASH-ROM" can not be read.
Therefore, boot-loader skips "SPI-FLASH-ROM", and boot from SD-Card.
However, in the clamped state, virtually "SPI-FLASH-ROM" is disable (it can not be read), so probably can not erase it.
I think that it is necessary to release the clamp promptly, after boot starts from the SD-Card.
What is the procedure you performed?
Sorry, it was a mistake
x "pin 20" -> o "pin 21"
Therefore, boot-loader skips "SPI-FLASH-ROM", and boot from SD-Card.
However, in the clamped state, virtually "SPI-FLASH-ROM" is disable (it can not be read), so probably can not erase it.
I think that it is necessary to release the clamp promptly, after boot starts from the SD-Card.
What is the procedure you performed?
Sorry, it was a mistake
x "pin 20" -> o "pin 21"