You need to try and understand why the voltage is dropping.

In my opinion from what you're written, the Hard Disk isn't getting sufficient power from the board to continue working and that's why it's spinning down.

From the limited information you've given, if the voltage is dropping and there is insufficient power to keep the HD spinning, you may have bad capacitors, because that's a typical symptom of bad caps.

That's just a stab in the dark from the information you've given - it could be a lot of other things as well.