unbootable usb stick "Non-system disk or disk error"

All, I have a USB stick that used to boot fine. I thought I'd reuse it. Reformatted it, copied some files over, and now it won't boot. I've used "lilo -M /dev/sdc mbr" with no luck. Note that the stick itself isn't busted, I can boot *to* it, but not *from* it but the computer refuses to like it as a boot device. Any idea what I broke, and how to fix it? As a backup it's usable so long as I boot it from a floppy, but I want it to boot directly like it used to. I erased everything off it in case the ext4 FS was somehow at fault (maybe the partition table or something), but no luck. However, if I 'dd' an .iso image to the stick, it boots to that just fine. I can then use lilo to write to the stick and have it boot to anything (ignoring the .iso), and all is well, *but* if I then delete the .iso, it's back to 'Non-system disk'. So it seems the .iso file is somehow involved in/with the MBR. Nothing like this ever happens with HDDs that I'm aware of. My understanding is that the MBR is a world unto itself and if it is 1st stage lilo bootable then it cares nothing about any other file on the disk until the 2nd stage is reached. I use floppies like that all the time (MBR only, no files at all) to control my boot. I can just leave the .iso there of course, and use partitions on the rest of the stick, but that seems very inelegant. Thoughts?