*snip*
I'm guessing that they way you've connected them, the computer is now
trying to boot from the other drive, and you haven't got anything on it
to boot from.  Try plugging them into each other's data ports, and see
if the system boots up from your first drive.  e.g. It's looking for
drive one, and you've got it plugged into socket two.

You might want to do what the message said, go into your BIOS, and play
with the options for selecting which is your boot drive.