Hmm, so it is same problem but card slots are switched. This is unfortunate. With N800 people rarely boot from external card with internal slot empty because internal slot is ideal for booting. Looks like with N810 mmc interfaces are switched so this bug will bite more people. What is interesting is that after the device boots the device name changes back. I remember when I booted system from external card 'df' command printed root partition being on mmcblk0p2 but FAT partition (on the same card) was named mmcblk1p1, do you see it too (if you still have FAT partition on internal card)? I am not sure where the bug is - kernel or udev or something else. Perhaps it is time to report it in Maemo bugzilla.