Is there a difference between the BIOS battery and the main battery?

the BIOS/CMOS battery powers the RTC (Real Time Clock) keeping the system date and time set correctly. I suspect what you mean by the main battery is the one that powers the whole notebook when it is NOT connected to a AC outlet.

To change the RTC battery you need to take the unit apart as it is inside and can't be accessed from the outside.

If you posted the problem you are having it would be a lot easier to help.

in addition to Ed's post perhaps you could also state what model laptop it is.

The notebook can't see the hard drive. It could be the hard drive has failed or the connection between the drive and the motherboard is bad. Take the drive out and put it back in to check the connection.

It is possible on some model Dells like EdC's and my Inspiron 8200 to disable the Harddrive in the BIOS boot order. If it does not have an arrowhead on the left end of that line entry, High light it with the arrow keys and press the space bar to enable it. If the Arrowhead is already there or your laptop has a different configuration, then what EdC said is likely to be true and forget what I said.

Thanks for confirming my conjecture, Bay Wolf. I knew it was true about the 8000-8200, but was not sure about the 8600

I have an I8600c and my BIOS has the option of disabling the hard drive.  It is on page 2 of 7.  Pressing the space bar will remove the arrow to the left of the hard drive.