Linux – Multiboot on usb with syslinux

I'm trying to make a multiboot usb with Ubuntu, Backtrack and Mantra-OS on it (all ubuntu as you can see). Those are the steps I've done:

sudo su
unmount /dev/sdb1                          # unmount the drive
mkfs.vfat -F 32 -n disqus /dev/sdb1        # Format the disk with vfat
syslinux -maf /dev/sdb1                    # Install syslinux and the mbr
mkdir /media/disqus                        # Mount for the disk
mount /dev/sdb1 /media/disqus
mkdir /media/disqus/syslinux               # Syslinux configuration folder
cp /usr/lib/syslinux/{vesamenu.c32,reboot.c32,poweroff.com} /media/disqus/syslinux          # Files needed for syslinux
mkdir /media/disqus/syslinux/iso           # A folder that will contain extracted isos
mkdir /media/disqus/syslinux/iso/ubuntu
mkdir /media/disqus/syslinux/iso/backtrack
mkdir /media/disqus/syslinux/iso/mantra-os

After that I mounted and copied files in the respective ISOs to folders I've made:

mkdir /media/iso
mount -t iso9660 -o loop,ro /home/nizar/Destkop/ubuntu.iso /media/iso
cp -r /media/iso/* /media/disqus/syslinux/iso/ubuntu
umount /media/iso/
mount -t iso9660 -o loop,ro /home/nizar/Destkop/backtrack.iso /media/iso
cp -r /media/iso/* /media/disqus/syslinux/iso/backtrack
umount /media/iso/
mount -t iso9660 -o loop,ro /home/nizar/Destkop/mantra-os.iso /media/iso
cp -r /media/iso/* /media/disqus/syslinux/iso/mantra-os
umount /media/iso/
rmdir /media/iso/

After that I filled on the configuration file syslinux.cfg in /media/diqus/syslinux with this:

DEFAULT Backtrack
PROMPT 0
allowoptions 0
TIMEOUT 100
UI vesamenu.c32

MENU TITLE Disqus: The Ultimate Disk
MENU BACKGROUND anon1.png

MENU COLOR border       30;44   #40ffffff #a0000000 std
MENU COLOR title        1;36;44 #9033ccff #a0000000 std
MENU COLOR sel          7;37;40 #e0ffffff #20ffffff all
MENU COLOR unsel        37;44   #50ffffff #a0000000 std
MENU COLOR help         37;40   #c0ffffff #a0000000 std
MENU COLOR timeout_msg  37;40   #80ffffff #00000000 std
MENU COLOR timeout      1;37;40 #c0ffffff #00000000 std
MENU COLOR msg07        37;40   #90ffffff #a0000000 std
MENU COLOR tabmsg       31;40   #30ffffff #00000000 std

MENU WIDTH 80
MENU MARGIN 10
MENU ROWS 5
MENU VSHIFT 10
MENU TIMEOUTROW 13
MENU TABMSGROW 18
MENU CMDLINEROW 11
MENU HELPMSGROW 16
MENU HELPMSGENDROW 29

LABEL Backtrack
    MENU LABEL Backtrack
    kernel iso/backtrack/casper/vmlinuz
    append initrd=iso/backtrack/casper/initrdf.gz file=/cdrom/preseed/custom.seed boot=casper quiet splash --

LABEL Ubuntu
    MENU LABEL Ubuntu
    kernel iso/ubuntu/casper/vmlinuz
    append initrd=iso/ubuntu/casper/initrd.lz file=/cdrom/preseed/ubuntu.seed boot=casper quiet splash --

LABEL Mantra-OS
    MENU LABEL Mantra-OS
    kernel /mantra-os/casper/vmlinuz
    append initrd=/mantra-os/casper/initrd.lz file=/cdrom/preseed/xubuntu.seed boot=casper quiet splash --

LABEL Reboot
MENU LABEL Reboot
COM32 reboot.c32

LABEL Power Off
MENU LABEL Power Off
COMBOOT poweroff.com

(anon1.png: 640×480 8bit png in /media/disqus/syslinux/)

Now everything is going alright and I'm able to boot, I have the syslinux menu and even load vmlinuz and initrd! The os is actually loading (I have the splash screen) but then I have the error (something about Busybox):

 (initramfs) unable to find a medium containing a live file system

I'm not quite sure what's the problem but I think it's about relative paths and the CDROM "thing". I've tested many of them (even relative and absolute) but nothing works.

The answer must not be about MultiBootUSB, MultiSystem, Yummi or any other tool; it must not be also about grub or grub2 or any other bootloader. I want to make my approach work.