This machine is relatively new and EFI based.
It will only display a list of bootable drives that are actually bootable.
Try booting from a CD/DVD/USB etc. and installing on to this drive and then you should get the ability to boot from the drive.
On some other machines, I have noticed that it will only list one drive as bootable no matter how many you have - The Bios/EFI treat "hard disk" as an option and you have to configure the priority of disks in a separate menu.
The last thing I can suggest/think of is if you have installed an OS on it already, the active parition/boot code may be on the primary drive and the menu was just reprogrammed to boot from the other disk.
I know I haven't really given much info to help you actually solve this issue, but this should kick you in the right direction - please comment back if you need any further help and I can tailor this answer better.