I am trying to dual boot my laptop between Slackware and Ubuntu. I was told to just use Grub to boot to the two OS's, so I had Ubuntu overrite the MBR with GRUB. This was able to successfully load Ubuntu, but it could not recognize Slackware, because I have an IDE based system. And Ubuntu, regardless of what type of hard drive you have, recognizes all drives on the system as SDA, not HDA. So Slackware wouldn't boot. So I am trying to configure Lilo to chainload to GRUB. How do I configure Lilo to chainload into Grub on /dev/hda2?

How Ubuntu recognizes drives is irrelevant. All you have to do is add an entry for Slackware to your GRUB menu.lst file. I infer that you have Slackware on hda1 and Ubuntu on hda2. So the Slackware entry would look somethink like this:
title Slackware
root (hd0,0)
kernel /boot/vmlinuz..... root=/dev/hda1 ro

(Assumes that there is no separate /boot partition.)

(Since you now have GRUB on the MBR, it's a lot easier to setup GRUB than to re-install LILO.)


More details in the "booting" link below.

You must reinstall Ubuntu's grub to the boot sector of /dev/hda2,
and (of course) reinstall Slackware's lilo to the mbr.

That will work - as will installing lilo to its own boot sector, and chainloading that from grub.
Only one can be setup to be in the MBR, not both.

Either way is easier than trying to directly reference the others kernel image - that would require a full path specification on the kernel and initrd.

I had GRUB installed before and it could not load Slackware, because it would come up with an error saying that HDA did not exist. I asked what that meant in the Ubuntu IRC chat, and they said it was because Ubuntu and GRUB with Ubuntu, recognize the HDD as SDA, not HDA.

That's because Ubuntu users don't know what they're talking about. (Just kidding, of course)

The problem was most likely due to the change in inode size from 128 to 256 bytes in ext2/ext3 filesystems in Slackware.
During the installation, you have the option of choosing an inode size. The DEFAULT inode size for ext2/ext3 filesystems is now 256 bytes in Slackware instead of 128 bytes (which is likely what Ubuntu uses). Slackware's GRUB available from extra/ from CD number 4 (or the DVD, or your favourite mirror) is patched to handle the 256 byte filesystem. There are two options. You could either reinstall Slackware from scratch and choose a 128 byte inode size for your ext2/ext3 filesystem (the stupid way), chainload Ubuntu's GRUB from Slackware's LILO (see wadsworth's post) or install Slackware's patched GRUB and use it to boot both Slackware and Ubuntu (the smart way -- though you must edit the menu.lst to add Ubuntu as a boot option).