Buying drive

I am thinking of buying a my book studio ii.  In the literature it says “simply open the case and replace the existing drive”.  What is being replaced?  Where do you get replacements?  Can this be used as a drive with removeable media?

It means that this external drive has 2 internal drive inside and if one drive fails you can just open the drive, remove the drive that failed and and replace it with a new one with the same model. This drive can be used as RAID.

I’m not interested in Raid.  Can I select which of the internal drives I want to write to so that an entire file say goes all to one drive?  If not I guess I would have to remove and replace the drives in pairs to maintain a coherent set of data?

Well you can also select drive as JABOD(just a bunch of disk) in settings. Which means that you can use the drives seperately as different drives.

Thanks!  Looks like what I want.