dd copy emmc for backup and use on other devices - gets input/output error
dd if=/dev/nbd1 …



/dev/ndbX is a Network Block Device. You're not backing up your EMMC, but something you've mounted via network; I'm almost sure that's not what you want to do.

I presume this means you run an NBD daemon on the embedded device, and mounted the shared volume on your PC. That is a possible approach, but I'd really just go for ncat here; run 

ncat -l … &gt; backup_image.img


on your PC, and 

ncat … &lt; /dev/mmblk0


on your embedded system (assuming mmblk0 is your EMMC).

Note: Many embedded systems do not run off self-managing flash devices that offer a block device interface such as MMC/SD cards/USB flash drives/SATA SSDs, but off raw flash ICs. These are NOT block devices (but MTDs) and don't offer a contiguous access method. Backing these up as physical layout to a file is hence (logically) not possible. Backing up the contents is.

If you'd try to export your MTD using a NBD daemon, things will go very wrong.