Creating a Linux Recovery disk

I am trying to create a Linux 5.7 recovery disk and am getting the error message below:

# createSRD --iso=/mnt/backup/rhel-server-5.7-source-dvd.iso -d /mnt/backup/customSRD.iso
Creating SRD...Please Wait
[ ########################                                            ] 36%
Could not get distribution info from media. Please check the media.
 

This would seem to imply a problem with the iso file but I have tried 2 different iso files and get the same error message. Anyone have any ideas on this one?

Thanks