Description of problem: When installing F7 using diskboot.img on a USB key, GRUB is installed on the USB key and not on the harddrive as expected. Meaning, that after the installation of F7 is complete it can't boot without the USB key is inserted.
This basically means that your BIOS is telling us that the first BIOS drive is the USB key when you're doing the install and so we default to using it. You can change the drive the boot loader is installed to either from the partitioning or the advanced bootloader screen. Unfortunately, there's not much we can do to better auto-detect things :(
But it worked in Fedora 6...