So is /dev/mmcblk1p2 existing according to sfdisk -l ?
If existing is it listed by sfdisk to be type 83 ?
If existing is it formatted ext3?
Is the mount point /mnt existing ?
Please to avoid simple mistyping errors, try to copy'n'paste in a code box.
It is easier to read and mor accurate to get the full error output to understand what is going on, aka why it is failing at your device while it works on others.