I have a USB key containing Ubuntu, BT and some other distros. Ubuntu and BT are peristent.

When switching to BT 4, BT now uses casper plus an initrd image to boot, like Ubuntu. It causes more pain to setup a USB key with two casper distro since the default way to setup a USB key is to put the casper folder at the root of the USB key.

On mine i have:
<root>
- ubuntu (contains all files of ubuntu-desktop.iso)
- backtrack (contains all files of bt 4 iso)
- syslinux (contains all config files for my syslinux menu)
- i have other distros also

With ubuntu 9.10, you can easily add a parameter to your syslinux entry to change the live-media-path location of the casper folder.

With BT 4, the live initrd image does not support it.

So i've written a blog article on how to unzip / patch / re-zip the initrd.gz file of BackTrack 4 to be able to pass this parameter.

Mycila Blog: BackTrack With Ubuntu USB Key

When editing the script/casper file, you can also change the name of the capser-rw partition for persistent change, so that you can have on the USB key:
- one partition for your distros and syslinux
- one XFS partition for Ubunto persistent named ubuntu-rw
- one XFS partition for BT 4 persistent named bt-rw