I get the following error during the boot process:
mount: mounting /dev/sda9 on /mnt failed: No such device or address
ERROR: mount returned error code 1. Trouble ahead.
mount: Mounting /dev/sda9 on /mnt failed: Invalid argument
/boot/initrd.gz: exiting

where /sda9 is my root directory.
My lilo config file, fstab and my /boot/initrd-tree seems to be correctly configured, however my load_kernel_modules file is empty (apart from a few comment lines), is that correct?
Support for ext3 was compiled as part of the kernel (2.6).

sdXX(sda9) indicates this is a SATA drive, inwhich case you need to compile SATA support into the kernel.
or build an initrd file containing the SATA module.
SATA modules differ by interface manufacturer, check your motherboard manual.

I included support for Sata into the kernel (two sata drivers I selected, one of them being VIA which is also the chipset of my mb). I checked all the kernel options before I compiled. I can't think of what I could have forgotten, (ext3 and sata was compiled in the kernel and not as module). I am also puzzled by the load_kernel_module file being empty.

What is the purpose of the load_kernel_module file,
That's the first time I hear of this, just curious

If sda9 is your root directory, why does it want to mount it
in mnt ? fstab error ?

Maybe it has a different name than sda

I was wondering about this file as it is empty, so useless. Do I have to fill it manually, can't imagine. I can't think of anything I did wrong in building the initrd-tree directory.
This is my fstab, I can't find anything wrong there either, and indeed why it wants to mount /sda9 on /mnt; seems rather nonsensical.

/dev/hda5 /__NTFS_DATA ntfs defaults 1 0
/dev/hda6 /__NTFS_GAMES ntfs defaults 1 0
/dev/sda9 / ext3 defaults 1 1
/dev/sda10 /var ext3 defaults 1 2
/dev/sda11 /tmp ext3 defaults 1 2
/dev/sda12 /opt ext3 defaults 1 2
/dev/sda13 /home ext3 defaults 1 2
/dev/sda3 /boot ext3 defaults 1 2
/dev/cdrom /mnt/cdrom auto noauto,owner,user 0 0
/dev/fd0 /mnt/floppy auto noauto,owner,user 0 0
devpts /dev/pts devpts gid=5,mode=620 0 0
none /proc proc defaults 0 0
none /sys sysfs defaults 0 0

In any case, if you compiled the root filesystem and
the hard drive driver as built-in in the kernel, you
don't need an initrd

So in a first step, try to remove the initrd line
in lilo, run lilo to save the change and reboot

I removed the initrd line and now I got another error, something similar I had earlier:
VFS: Cannot open root device "809" or unknown-block (8,9)
Please append a correct "root=" boot option
Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block (8,9)

Do you have SCSI disk support compiled as built-in ?
(Device Drivers > SCSI device support)

I'll check tomorrow, but I am almost sure I have.
I've always prefered Slackware, but this time it gives me the impression I don't understand much of it.

Take yourself some time, it is not slackware fault

Hardware becomes more and more sophisticated and
then the linux kernel too, so you need to take
some time to learn to enable the right things in
the kernel compilation

I checked scsi support in the compile options and this is what I had:

SCSI device support <m>
legacy /proc/scsi/support
scsi low level drivers:
serial ata support <m>
VIA sata support <m>

Support for Sata (deprecated: conflicts with Libata sata driver) I didn't select instead I selected the above options as the sata driver was now part of the scsi controller, so I understand, although reference to the Libata driver I didn't find in the helpsection.

I rebuilt my kernel and modules. I replaced the modules above by in-kernel built (as I did with filesystem support).
This resulted in a kernel panic error similar to what I had previously.

My load_kernel_module file was empty again. I was wondering whether it should contain the contents of the file modules.conf which was configured by alsaconf, although that has no priority at this moment.

I built the initrd-tree as follows:
mkinitrd -f ext3 -k 2.6.17.13 -r /dev/sda9
I don't know what's going wrong here and there is nothing special about my hardware.

Hey make your life easy, forget the initrd

Instead of configure the SCSI device support as
module (<m>), configure it as built-in (<y>)

Do the same for serial ata and via features.

Then edit your lilo.conf, remove the initrd line
save it, run lilo and try a boot

That is what I did and wrote in my previous mail. Those compile options given I built in the kernel, * instead of m
When I removed the initrd section this time, I got the following error:
VFS: Cannot open root device "<NULL>" or unknown-block(8,9)
Please append a correct "root=" boot option
Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block (8,9)

The bootloader is in the root section, /sda9, not in the MBR. Could that make any difference?