/dev/mmcblk0 is always the first memory card:

• in the case of the 770 it is the only removable memory card
• in the case of the N800 it is the card in the battery bay
• in the case of the N810 it is the 2GB internal memory

/dev/mmcblk1 is the second memory card:

• in the case of the 770 it doesn't exist
• in the case of the N800 it is the SD card under the kick-stand
• in the case of the N810 it is the miniSD card under the kick-stand

When cloning on 770 and N800, my choice is to clone the OS to /dev/mmcblk0p2 (ie. second partition on the first memory card aka the internal card).

When cloning on N810, my choice is to clone to /dev/mmcblk1p2 (ie. second partition on the second memory card aka the external card).

The above advice has worked for me every time. My updated cloning script should automatically identify and use the correct memory card and partition (internal or external). There is a bug in the kernel which Fanoush referenced elsewhere which causes the memory card devices to be incorrectly identified which is annoying so be careful when viewing /proc/partitions as it gives bogus information.