Reboot and Select proper Boot device or Insert Boot Media in selected Boot Device & Press a Key

First off, one little bit of information that might help is if the hard drives have a SATA interface or an IDE interface.  The second thing you want to check is if the jumper caps are set correctly on teh hard drives.  If you have two hard drives, then it is best to follow the configuration on the hard drive so the hard drive with the operating system is the master and have the second hard drive be the slave.  Because if both hard drives are set to master or both are set to slave, problems occur.

Okay then.  Most hard drives are set up so right next to the power socket is the pins for the jumper cap.  Somewhere on the hard drive should also be a sticker label that shows diagrams of different possible jumper cap combinations and what they mean.  There should be one configuration for master, one for slave, and another for cable/computer select.  These options might be abreviated on the lable.  Then look at what configuration your hard drives currently have.  Post them here if possible.  But basically look and make sure that one is master and the other is slave.  If it is on computer/cable select, then I am not really certain because technically the computer is supposed to be smart enough to figure out which hard drive to boot to, but I don't trust it and I always set one to master and the second to slave.  Just make sure the hard drive with the operating system is the one you set to master.