octathlon, here is a page that describes what GRUB Error 18 means:
Briefly, GRUB Error 18 means "Selected cylinder exceeds maximum supported by BIOS" and it can be fixed by "creating a boot partition at the beginning of the disk that is completely within the first 1023 cylinders of the harddrive."
It's a little bit difficult to diagnose since I don't know all the details of your system. Do you have two different physical disk drives on your system? That's what I'm thinking based on your "slave drive" and "master" description. If that's the case, then is your first partition on the slave drive larger than 1023 cylinders in size? If so, that's the problem. You'll have to repartition that slave drive to get the size of the first partition down to 1023 cylinders or lower.
Feel free to repost here, or send an email to me. But posting here might help future readers who come across the same problem (GRUB Error 18 is apparently pretty common)...