I also bought emmc2microsd but there is a problem with small notch on it. My emmc plug in oposite way and than there is no way you can plug microsd anywhere. After a while I forced it in oposide way and was able to erase emmc with PC.
There was no chance to stop booting to go to U-boot. It simply does not work..
But I manage to solve this probem..by force..