Before I recently simplified my N800 to boot from my internal slot in addition to flash, I had set up my N800 to boot from both my internal and external slots, so as to be able to run KDE on one and OpenBox Rox on the other. Fanoush's bootmenu flashing application (initfs_flasher) gives a sample bootmenu.conf detailing just how to boot from either internal or external card. I suggest you search this out and get back to me (or better Fanoush) with any further queries. Good luck.