Confused about Core question....

I'm taking my Core exam on the 19th of May, so I'm getting all the practice as I can I recently came across a question I've seen in the past and I'm very confused on it. I need help with this one.
In a PnP system when installing a second hard drive what determines the drive letter?
[removed by webmaster: don't post other sites' questions...]
In a PnP system when installing a second hard drive what determines the drive letter?
[removed by webmaster: don't post other sites' questions...]
Comments
Take 1 hard drive and put it into any machine and turn the machine on. What happens?
At best the BIOS will detect the hard drive at worst nothing or an error.
You are going to need some form of software to partition, format and use the disk. Therefore if I may be so bold as to suggest software would be the answer. You can now argue which bit of software.....