either my search techniques are awful, or it isn't yet completely described:
how does one create an exact backup of the entire filesystem to mmc card?

what I have is :
#mount -t jffs2 /dev/mtdblock4 /opt
#./mkfs.jffs2 -r /opt -o /media/mmc2/rootfs.jffs2 -e 128 -l -n
/bin/sh: ./mkfs.jffs2: not found

I am running OS 2008 from an n800. any suggestions?