@rich; No, i was writing to /mnt/sda2.. but i'm guessing this was just a placeholder folder and nothing was actually mounted-- everything written disappeared on reboot.
now, i have a knock-on problem.. from trying to set boot options 'opt=sda2' and 'home=sda2' (which have since been un-set) i now have /boot, /home and /opt directories on /dev/sda2, and i can't delete them[e: just /boot i can't delete] (permission denied). >??