Reported by Nikolaus Schaller, Jun 3, 2014

Adressing individual partitions from raw device path needs to check 
for:

/dev/sd* -> ${DEV}1, ${DEV}2 ...
/dev/mmcblk* -> ${DEV}p1, ${DEV}p2 ...

We can't check [ -b "${DEV}p1" ] before creating a 
partition table on a completely erased SD card. So we must match 
patterns.

And "unmount all" should be done as

for partition in ${DEV}*; do umount $partition; done

before running fdisk.