I recently replaced my drives and came across the same situation. It’s true the OS is stored on the hard drives that came from the factory, BUT it is also stored on the firmware chip on the ShareSpace unit and it will install it onto new hard drives so that you can use them. The trick is you have to zero out the hard drives first.
Here’s a step-by-step guide I made to help out others:
http://niksideas.blogspot.com/2012/01/wd-sharespace-nas-installing-new-hard.html
I successfully installed 3 old WD hard drives I had laying around into my ShareSpace using this method.
-Nik
