NTLDR is missing when trying to boot from USB drive

NTLDR is missing when trying to boot from USB drive

I've just followed these instructions to make my USB HDD bootable, to install Server 2008 R2, but when I set my BIOS to boot from USB HDD, I get the error "NTLDR is missing". I did copy all the Server 2008 R2 files over from the ISO after mounting the ISO.