I think isohybrid is key.  I use a modified script originally written by 
Connie Sieh (thank you!).  I can certainly pass it along if you are 
interested.

Instead of copying the files out of the ISO, I just mount it on loopback 
and graft what I need in with mkisofs: (some snippets)

(mount netinstall ISO to $TMPDIR_ISO)

cd $TMPDIR_ISO/
#Determine toplevel directories from mounted original CD
# We ignore isolinux, EFI, and Packages
TOP_CD_DIRS=`find . -maxdepth 1 -type d -print | sed -e "s/.//" | sed -e 
"s,/,," | sed -e "s/isolinux//" | sed -e "s/EFI//" | sed -e 
"s/Packages//" | sed -e "s/repodata//"`
for DIR in $TOP_CD_DIRS
do
   GRAFT_DIRS="${GRAFT_DIRS} ${DIR}=$TMPDIR_ISO/${DIR}"
done


cd $TMPDIR_NEWBASE/
/usr/bin/mkisofs -v -U -J -R -T -m repoview -m boot.iso -b 
isolinux/isolinux.bin -c isolinux/boot.cat -no-emul-boot -boot-load-size 
4 -boot-info-table -eltorito-alt-boot -e images/efiboot.img 
-no-emul-boot -graft-points \
   -V $VOL_ID \
   -o $WORKDIR/$CONTEXT_NAME/$CONTEXT_OUT_ISO \
   $TMPDIR_NEWBASE \
   $GRAFT_DIRS \
   contexts=$WORKDIR/$CONTEXT_NAME/contexts \
   repodata=${NEC_BUILD_REPO}/repodata \
   Packages=${NEC_BUILD_REPO}/Packages \
   elrepo=${NEC_BUILD_REPO}/elrepo \
   epel=${NEC_BUILD_REPO}/epel

(some cleanup/unmounting here)

/usr/bin/isohybrid -u $WORKDIR/$CONTEXT_NAME/$CONTEXT_OUT_ISO
/usr/bin/implantisomd5 $WORKDIR/$CONTEXT_NAME/$CONTEXT_OUT_ISO



On 05/24/2018 01:52 PM, Kraus, Dave (GE Healthcare) wrote:
> Pungi uses isohybrid (from the syslinux package) down in the bowels to do that. At least that's what I found when I deconstructed its methodology.
> 
> That's how I'm getting a USB-bootable .iso file for our spin, anyway...
> 
> The algorithm I'm using is:
> 
> 	mkisofs
> 	isohybrid
> 	implantisomd5
> 
> Your mileage may vary...
> 
> On 5/24/18, 1:35 PM, "[log in to unmask] on behalf of Pat Riehecky" <[log in to unmask] on behalf of [log in to unmask]> wrote:
> 
>      Hello,
>      
>      Are you creating the custom iso with pungi or a different tool?
>      
>      Pat
>      
>      On 05/24/2018 01:17 PM, Bill wrote:
>      > I am creating a custom installation ISO using kickstart.  This install ISO is based on SL 7.2.
>      >
>      > When I burn a DVD from this ISO I can boot from the DVD and the install menu come up as expected.
>      >
>      > When I use dd to copy the ISO to a USB key and try to boot from the USB key the install menu does not appear and the system boots from the hard drive.
>      >
>      > I tried using dd to copy the SL7.2 ISO to a USB key.  When I try to boot from this USB key the install menu comes up as expected.
>      >
>      > What files on the SL7.2 have to do with booting from the USB key?
>      >
>      > Is there a mkisofs option I should be using to make booting USB key work?
>      
>      --
>      Pat Riehecky
>      
>      Fermi National Accelerator Laboratory
>      www.fnal.gov
>      www.scientificlinux.org
>      
>