Ok you have a Bucks Ahoy that's a start.
Next thing we need to know is the top box bios type.
When the machine boots up does the top box display a red and blue finger (bios 1) or does it display a green and yellow energy star in the top right of the screen (bios 2).
Next things we will need to know is the MPU type. 500 series or enhanced type.
We also need to know the SB, SG, and VS chip numbers on the MPU board.
You will also need an old computer keyboard with the small ps2 plug (purple) usb keyboard will not work, if you plan on changing the top box battery.
Try to change the battery in the top box. Turn off replace the battery plug in the keyboard on the side of the top box housing. Then turn on and keep hitting the delete key until it enters the bios screens. Set the time and date then save also "load optimized defaults " save the bios. Turn off, unhook the keyboard and if the hardrive is good it should work. CD rom useless unless you have load disks.
Chances are the hardrive is bad as you have stated.
Rich