I was installing CentOS on to a disk that already had a partition. The installer wanted to install a PReP partition which it couldn't as the first 4GB of the disk were full.
As I didn't need a PReP partition (because the machine has Petitboot) and I didn't want to rearrange my partitions, I pressed the "Do not install boot loader" as this was the only way (that I could find) that didn't require me to have a PReP partition to install CentOS.
Turns out that doing this means that the installer doesn't update the grub.cfg file. As Petitboot looks for this file and it isn't there, it doesn't display the new OS on the boot menu.
One fix is, after installation and before rebooting:
- go to the anaconda shell (press ctl+b then 2)
- update grub.cfg:
grub2-mkconfig -o /boot/grub2/grub.cfg