First, is the
ks.cfg file valid? Run
ksvalidator to check for mistakes in syntax/formatting.
If you are modifying the boot media to include your own custom Kickstart, you'll need to copy the contents of the media to local disk to make your changes. It sounds like you are trying to copy your script to non-writable media such as a CD-R/RW. Otherwise, it's as simple as making the existing ks.cfg writable on disk. (
chmod ug+w ks.cfg)
Here are my complete notes on customizing boot media. (I start with an ISO I've downloaded from Red Hat instead of existing media.)
Copy vendor media to disk.
mount -o loop /path/to/boot.iso /tmp/bootiso
cp -r /tmp/bootiso/* /tmp/bootisoks/.
Make ks.cfg writable and overwrite the vendor provided.
chmod ug+w /tmp/bootisoks/isolinux/ks.cfg
mv /path/from/ks.cfg /tmp/bootisoks/isolinux/ks.cfg
Make new ISO from source.
mkisofs -o /path/to/modified.iso -b isolinux.bin -c boot.cat -no-emul-boot -boot-load-size 4 -boot-info-table -R -J -v -T isolinux/
Then, you can burn the new ISO to disc and test a Kickstart!