USB booting :: Non system disk?

For some reason, when I try to boot to my usb with DSL on it, it's not seeing it as a bootable disk. It's giving me a "Non system disk" type error, similar to what you used to get when there was a floppy with no OS on it. My computer does support USB booting, and I've done all the steps given to put DSL on the USB (mounted the ISO and booted from CD-ROM and used the command inside to mount to the USB). I'm not getting any errors when I'm putting it on the USB within DSL, but again, my computer is saying it's an invallid disk when I try and boot via the USB.
I've tried this with two different USB drives with the same result. Any ideas? Any help is appreciated.

original here.