Thank you so much MrEngman! I've been having the same problem for a while now, and this is such a necessary step for having a hard-to-access Pi like mine. Now I don't have to run outside and reboot the thing to tweak it.
I have a similar problem. My 4GB SD card is essentially full, but I have many free external hard drives. How would I set the system up to boot from the SD card but have everything else on an external hd?