It sounds like you had an "oxidized/loose" connection on one or both hard drives. I doubt it is the BIOS battery, if it now boots, and you can check the settings in the setup BIOS. If all there is normal, it most likely is NOT the internal backup battery. (If the battery were bad, you would have lost your HD parameters, and other settings, and the machine would have to re-establish those settings with each boot.) But keep an eye on it nevertheless; there could be a loose connection elsewhere. Have a spare power supply available if you can.