So, my grandpa recently got an iMac to replace his old HP desktop, because the motherboard melted a little bit. The hard drive from the old was still good (or so we thought), so I tried to see if I could boot with it. With the HP hard drive attached via USB, I tried to open the Boot menu by holding the Option key at startup. That didn't work (with our third-party keyboard OR the original Apple keyboard. So I did it through System Preferences -> Startup Disk, selected the HP drive and restarting. It didn't work, something's probably wrong with the drive. Oh well. So I shut the computer down, unplugged the drive, turned it back on, and... it's still trying to boot from the HP drive that is no longer connected. On startup, it makes the standard startup sound and shows a blank grey screen. After a little while, the screen goes black and this text shows up: "No bootable device -- insert boot disk and press any key _ So, what do I do? How do I get it to boot from it's own hard drive again? Just as before, holding Option through startup fails to bring me to the boot menu. I've also tried unplugging everything, including the power cable, for a little while and trying again, that didn't work either. Help!