I have a multiboot usb with System Rescue CD and GRUB2 on it. When I try to boot it, it tries to find systemrescuecd/sysrcd.dat, attempts to mount /dev/sr0 and all the partitions on /dev/sda before declaring cannot find systemrescuecd/sysrcd.dat on devices and dumping me onto a primitive shell. The relevant entries in grub.cfg:

menuentry "SystemRescueCd 32bit" {
linux /systemrescuecd/isolinux/rescuecd rootfs=/systemrescuecd subdir=systemrescuecd dostartx setkmap=us
initrd /systemrescuecd/isolinux/initram.igz
}
menuentry "SystemRescueCd 64bit" {
linux /systemrescuecd/isolinux/rescue64 rootfs=/systemrescuecd subdir=systemrescuecd dostartx setkmap=us
initrd /systemrescuecd/isolinux/initram.igz
}

I think the problem is that System Rescue CD cannot see /dev/sdb, which is my usb, but I don't know where to begin to fix it. If it helps, I set up my USB with a utility called MultiSystem, which is like MultiISO for Linux.