A possible of Grub 'Error 18' and Grub2 'Out of disk' errors is that you are running with a rather old BIOS and your kernel image lies beyond the 1024 cylinder limit.
Older BIOSes use CHS rather than LBA to address the boot disk. The disk address of the kernel that is stored in the boot sector has only 10 bits for the cylinder number. Therefore the kernel and initrd, if used, must lie entirely below the 1024 cylinder boundary. Failure to observer this restriction gives rise to the error.
To prevent this occuring create a /boot partition of, say, 100MB at the start of the disk and store your Grub configurations, kernels and initrd images in it. You can then create your root partition where you like, even on a different disk.