backtrack iso booting from grub2

["J. Bakshi" <bakshi12@gmail.com>]

Dear list,

I have live usb system where I already have debian, puppy, ubuntu etc.....
I like to boot backtrack iso from this usb drive. 

Here is the entry for BT5 I have added at grub.cfg

```````````````````````
menuentry "BT-5" {
loopback loop /BT5-KDE-64.iso
linux (loop)/casper/vmlinuz  BOOT=casper boot=casper iso-scan/filename=/BT5-KDE-64.iso  nopersistent noprompt --
initrd (loop)/casper/initrd.gz
}
```````````````````````````

but grub throws errors as

```````````````````````````
error:can't read linux header
error:you need to load linux kernel first
`````````````````````````

Any idea what is missing here ? I have already checked the iso by mounting as loop device
and confirmed the presence of casper/vmlinuz and casper/initrd.gz. So why is it complaining ?

Thanks