and as your music could be anywhere on any of your drives, it has to spin them up to check. Just because your library is on the internal drive, it does not necessarily follow that the actual tracks are there too.

You will find that it does the same thing if you open a document, in Word for example, and then choose "Save As" when the drives are not spinning.
You will not be able to do anything until all the drives have come ready.

P