[CentOS] USB of ISO image

Hi

I did the dd iso to my USB. all is fine.
I can mount the second partition (which is not iso 9660) and edit the
grub.cfg file.
Question is what do I run after that so my new menu option appears ?

grub2-install /dev/sdd


Is that the correct command to get grub to notice my new file ? Assuming
sdd is the USB disk of course.

Thanks,

jerry