You have a boot drive: it is your WD Black. A boot drive is simply the drive containing the partition containing the operating system. That is, the location of the instructions read into the computer which load the operating system. So, yes, one is mandatory, but it may also contain more than the operating system. It still is your boot disk. It is possible to put the operating system onto a very fast drive such as a Flash Drive. This will dramatically speed up loading the operating system. Depending on what applications you use it may also speed up the operation of those programs.
Larry