USB booting :: Grub Error=15

( my home comp is too old to boot from usb)...is this error a sign of an incompatibility, or a fixable error?
Thanks!

Search: http://www.gentoo.org/doc/en/grub-error-guide.xml#doc_chap4 says file(s) not found.

What are you trying to do, etc.? Please give more details, and info on your grub config.

original here.