Your grub just needs to know how to read the media, and you can specify any installed disk as the source of the boot files. That grub is loaded from a USB stick is really irrelevant. The kernel and initrd are specified as usual, and the location from which they are loaded is the root media:
Code:
# hd0 and partition number can be any valid media
root (hd0,0)
There may be some chicanery required to ascertain what media is assigned the the various hd numbers, but that is all just system-specific configuration.
--- rod.