I've tried several different computers and two different (brands) of usb drive. Just to make it supremely aggrevating, it did actually boot once, successfully. The only thing I've noticed (when turning off "quiet splash" in syslinux menu) is that it complains that /lib/modues/2.6.24-16-generic doesen't exist. And that's because it doesn't--2.6.24-19-generic does. But I've no clue where this path is called from, nor why it would have succeeded even once if this were really a fatal error.

Any ideas?

Thanks.

You would typically get that kind of error when you have multiple controllers and your OS cannot figure out which one to boot. It is and old - and I agree, irritating - issue that Ubuntu seems to have inherited from Debian, where it has existed for several years now. If you check their site, you will find the bug listed with the following advice: "just reboot or reorder your drives - but we can't guarantee that it won't happen again".

The only failsafe solution I can think of is making sure that your USB drive is uniquely identified. You can use UUID, disk labels, ... - anything except the traditional /dev/sdX thing - because that's where things go wrong: what is sda on one boot may be seens as sdb on the next one, depending on which controller gets picked first.

Oh...uh...how do I do that? UUID, that is.

Thanks for the info, btw.

You would use this command to find out the UUID of the device in question:

Then you need to delete the /dev/sdX part(s) from both /etc/fstab and your grub menu.lst and replace with the proper UUID.

You could also use a label. If you use ext2/3, you would use:
for example, sudo tune2fs - L Ubuntu_sys /dev/sda1. This will name the partition Ubuntu_sys, just like you can give windows partitions a name. Then it's the same procedure as above, remove the /dev/sdX from /etc/fstab and boot grub menu.lst and replace with LABEL=Ubuntu_sys. Do NOT do this on a mounted system!