obscure hard drive question

I installed a new hard drive a few days ago. It works great, except when I first boot up the computer it says it can't find the system disk. If I hit the reboot button the computer boots up fine. It then reboots as normal after that until I shut it down. Its no big deal because the computer is working with no problems, but each time I start up the computer I have to reboot- which is a tad annoying. Any ideas...?

Thanks!