[CentOS] Installing from a usb drive

Hi, all.

We've make custom isos of CentOS and having trouble during the installation.

Here's what we did :

Add our rpms to CentOS/RPMS
Add a folder of scripts and two kickstart files to the base directory
run "pkgorder /path/to/base i386 CentOS > /tmp/foo.txt"
run "genhdlist --fileorder /tmp/foo.txt /path/to/base"
rerun the pkgorder command
Add a custom group to CentOS/base/comps.xml
run "buildinstall --pkgorder /tmp/foo.txt --version 4.2 --product "CentOS"
--release "BAR_V1" --discs "BAR" --prodpath CentOS --comp BARv1
/path/to/base"
change isolinux/isolinux.cfg to add two labels, one for each ks file
run "mkisofs -o foobar.iso -b isolinux/isolinux.bin -c isolinux/boot.cat
-no-emul-boot -boot-load-size 4 -R -J -v /path/to/base" to generate the
DVD
iso.

This iso works fine when we burn it to dvd and install from it but
when we boot from DVD and select the kickstart file that installs from
a USB drive, we get the following error :

"trying to mount already-mounted image!"

Could anybody explain why ?

Emmanuel