I guess it's nearly a FAQ but the several step-by-step methods I followed
didn't grant me success.

Here's what I did:

1) Install a 3.5" IDE drive in an external USB enclosure;
2) Install Slamd64 12.0 on this drive;
3) edit lilo.conf, /sbin/lilo
4) reboot and boot off this device

I got a kernel panic: unable to mount root device.

After searching on the net, it seemed the easiest way was to create an
initrd that would have the USB drivers inside.

I booted off the slamd64 first CD; I recompiled the kernel to have ehci-hcd,
uhci-hcd, ohci-hcd and usb-storage as modules; through chroot this kernel
was built and installed on the external drive.

Next I went into /boot, and did

1) mkinitrd -c # to start afresh
2) edit initrd-tree to have a 30 sec. sleep before "Insert LVM"
3) mkinitrd -k -m ehci-hcd:uhci-hcdhci-hcd:usb-storage
4) /sbin/lilo

but still no go: on booting I still have a kernel panic because /sbin/init
couldn't be found.

Anybody to help me? Thanks.