I set up my n800 with the latest firmware to boot from the MMC card following the wiki instructions. I rebooted successfully, and verified that it was really running from the card (/ was mounted on /dev/mmcblk1p2). Ran it that way for a few hours, then removed all cards, reflashed the device with the original firmware to be able to use Kismet, got kismet up and running (runs great on original firmware, with GPS!), and then re-installed the mmc boot menu program, and the MMC cards. Now I get the boot menu, choose the partition, and it says it "cannot boot from MMC, booting from flash". The cards are back in their original slots, and when I mount /dev/mmcblk1p2, I can see the whole filesystem, so I didn't lose anything.

I did not make a copy of the original firmware to the MMC card, as I already had a copy of the new firmware on it, and wanted to be able to dual-boot between the firmwares when I wanted to run kismet vs. have nice Flash video performance. Is such a setup possible? Can I effectively dual-boot the n800? Or is there something that only allows multiple copies of the same firmware? I know on the Zaurus with altboot, it only loaded userspace stuff from the MMC card, the kernel was loaded from NAND flash, and it would complain if your ROM image was not correct for the kernel version stored in flash.

I don't know where to start troubleshooting, any advice would be appreciated.

Yes kernel is shared so the best is to use same firmware. Some combinations work fine, some not. I guess newer kernel works with older rootfs on mmc card, not the other way around (which is what you are trying?).

When booting from external card make sure you have both cards inserted on boot. If you have only external inserted kernel is confused at boot time and thinks it is the internal one (so the internal card menu item works). This may be the reason for the message you see. if you have both cards inserted and not swapped by mistake, I don't know.

But anyway newer rootfs with kernel from older firmware will cause problems.

Nope, trying it the other way around - new rootfs on MMC, older firmware/kernel on MMC, so I guess it's no-go.

Both were inserted.

Yep, sounds like I'm out of luck, but I will try the reverse, that is, clone the old OS that's currently on the device to MMC, then flash to new version in flash memory, and see if it will boot that way. I'll report back if it does.

Yes, it works, I have tried, but if kismet depends on specific older version of kernel or wlan driver (=your reason to use older firmware?) it won't work since you'll get newer kernel and wi-fi driver.

See, that's what I'm not sure of - nor does anyone else seem to know what's keeping kismet from running on the new firmware. I would guess it's a userspace issue since kismet fires up and sniffs for a few seconds, then just stops, but it at least runs.

Also, there's another driver that can be used.

Well, it boots with latest firmware in flash, and older firmware on card, but no luck running Kismet while booted into older firmware from MMC. Tried with both the stock and open source driver, but it still stops scanning. Guess that probably means some sort of kernel issue. Nice to know though that dual-boot is at least somewhat possible for other apps.