I had a similar problem
I received a message that I needed to download a bios upgrade for my new satelite laptop not even a year old. After download and install the hard drive itermintantly runs when the laptop is idle. I cannot get anyone to fix the issue. I am willing to go back to the old bios but don't know how to do it.
Drive running when the laptop is not working is usually the indexing the search feature does. Could also be downloading MS updates. Has this been happening for a while now and constantly?
BIOS updates are normal, usually happen a few times during a computer's life span, nothing to worry about. I have not actually seen a BIOS that would allow you to go to an older version, you always get the message "your current bios is the same or newer" and the setup exits. You can try to contact the PC vendor support and see if they have any work-arounds for this.