I've rerun the sha256sum and it is still okay.
I wasn't sure whether you meant to replace the conv arg or add, so I tried it both ways.
Code:
# dd if=/dev/zero of=/dev/mtdblock6 bs=512 conv=noerror
dd: can't open '/dev/mtdblock6': No such device or address
# dd if=/dev/zero of=/dev/mtdblock6 bs=512 conv=fsync conv=noerror
dd: can't open '/dev/mtdblock6': No such device or address
I might just retire this device as opposed to use a workaround to make it work.
It was just something that peaked my interest, but that is quickly waning.
Thanks for your help (and all others that contributed).