I've got a usb-key setup to multi-boot different Linux images. I'm trying to add CentOS, here's a [post with similar entries](http://thermsnotes.blogspot.com.au/2011/11/grub2-loopback-multiboot-usb.html) for `grub.cfg`

```
set drive_label=multipass01
set isopath=/iso

menuentry "CentOS-6.3-x86_64-LiveCD" {

# Tried the following root designations;
# root=live:LABEL="multipass01" 
# root=live:LABEL=multipass01 
# root=live:LABEL="multipass01":$isofile
# root=live:LABEL=multipass01:$isofile
# root=live:/dev/disk/by-label/$drive_label$isofile
# root=live:UUID=3A55-12BC:$isofile
# root=live:UUID=:$isofile
# root=UUID=3A55-12BC
# root=live:$isofile
# iso-scan/filename=$isofile

    set isofile="$isopath/CentOS-6.3-x86_64-LiveCD.iso"
    loopback loop $isofile
    linux (loop)/isolinux/vmlinuz0 boot=isolinux root=live:LABEL=multipass01:$isofile rootfstype=auto ro liveimg 3 rhgb rd.luks=0 rd.md=0 rd.dm=0 splash --
    initrd (loop)/isolinux/initrd0.img
}

```

Heres the result of attempting to boot this entry (regardless of which `root=`);

![kernel panic](https://i.stack.imgur.com/1BN7T.jpg)

edit; [related fedora bug](https://bugzilla.redhat.com/show_bug.cgi?id=723801)