change this

mage = /boot/vmlinuz
label = SuSE
root = /dev/hdc3
vga = 791
initrd = /boot/initrd
append = "hdb=ide-scsi"

to this

mage =/mnt/hdc3 /boot/vmlinuz
label = SuSE
root = /dev/hdc3
vga = 791
initrd = /mnt/hdc3/boot/initrd
append = "hdb=ide-scsi"

if you run lilo from Debian, your Suse vmlinuz etc are in
partition /mnt/hdc3/boot no in Debian /boot
If you run from Suse Debian partition is in /mnt/hda3/boot etc etc
I dont know Suse and /mnt/hda3 may be /mnt/hda3 or /mnt/anything