OK, so I managed to get my N800 booting from an external SD card, and it's been working fine for weeks. I took the SD card out of my N800 and popped it into a reader on my Linux box (fresh Jaunty install). For some reason, Jaunty wouldn't automount the card, so I just mounted it from the command line. I fiddled with some audio files and created an m3u file, then umounted it from the CLI and put it back into my N800.

Whatever I did screwed up my boot, as now the N800 just boots from the internal flash. It doesn't look like it even sees the SD card. HELP! What do I do? I've done an fsck on both the ext2 and vfat partitions, which repair a bad unmount I did on the ext2 partition (my linux newb is showing!), but that did not restore my external boot.

is it a real full sized SD or a mini/micro+adaptor?

we've had cases of broken adaptors here: card totally OK but not recognized by tablet because of adaptor.

Full-sized SD. It looks like the N800 actually sees one of the partitions (not the main one with all the data), but something's gone wrong with booting from it, as the bootmenu doesn't even appear.

I managed to fix this, but not sure how. I switched the SD card from the internal slot to the external slot. Doing so gave me the boot menu, but booting from the external slot failed. I switched the SD card back to the internal slot, and viola! the boot menu had returned.