Modified ISOs are unbootable

Friends,
I'm trying to create some menu options for our boot image (for a 
networked KS install), and for some reason I'm not able to make a 
bootable ISO. Here's the minimum I can do to recreate the problem:

:) wget 
http://mirror.itc.virginia.edu/fedora//releases/18/Fedora/x86_64/os/images/boot.iso
:) mkdir mnt
:) mkdir copy
:) mount boot.iso mnt -o loop
:) cp -avr mnt/* copy
:) mkisofs -o boot2.iso -b isolinux/isolinux.bin -c 
isolinux/isolinux.cat -no-emul-boot -boot-load-size 4 -J -R -V "Fedora" copy
:) dd if=boot.iso of=/dev/sdc
:) dd if=boot2.iso of=/dev/sdd

Okay, so I did absolutely nothing to the image, right? But...
:) ll -h
   351M boot2.iso
   294M boot.iso

And, most importantly, boot2.iso doesn't boot at all - like, the bios 
says "requested boot device unavailable" or something. (Dell Precision 
T3400)

So, uh, if you wanted to edit isolinux/isolinux.cfg to add menu options 
for a network kickstart, how would you do it?

Thanks much,
Charles.



More info:

mkisofs options grabbed from 
http://techspotting.org/creating-a-kickstart-cd-dvd-fedora-redhat-centos/


:) isoinfo -d -i boot.iso > b1
:) isoinfo -d -i boot2.iso > b2
:) diff b1 b2
   3c3
   <Volume id: Fedora 18 x86_64
   ---
   >Volume id: Fedora
   14C14
   <Volume size is: 150207
   ---
   >Volume size is: 179536
   29c29
   <Bootoff 3567 13671
   ---
   >Bootoff 2F 47

If I mount boot2.iso, it is identical to /mnt as far as I can tell. 
(same size, same dates, same permissions...)