On a stock 5.2 to 6.0 upgrade on a low disk space box, the installer failed to install lilo correctly. This was either due to running out of space (with no error message - there's another ticket on it) or because the kernel was complaining about a bogus hdb geometry. In that box, hda hdc and hdd are all small hard-drives. (hdb doesn't exist at all). On the kernel boot up, it complains, but by looking at f3 or f4, the install boot loader portion also doesn't like this.
We will need to know what the LBA geometry of the drives as reported by the BIOS and what the kernel is reporting when booting. You may need to give the kernel specific geometry parameters at the lilo boot prompt to override what the kernel is detecting.