I think there may be an issue with the Mac (see here
), though over there it is with Lion. Unmounting was also a problem before, solved apparently jsut by pulling the cable out
As for the "missing" space, there is one GB that is used by the device adn that we cannot see: but you can access it if you use one of the apps developed on this forum, see the sticky on the Pro models, there are links there. I do not know why is it that you cannot put the books back, perhaps they were in the non-accessible part of the memory? But I am no techie, so perhaps this makes no sense.
As for sluggish behaviuor: the current firmware now indexes the library contitnuously, so it could also be that taking books out and putting them back is slowing it down (but you can change this setting in the configuration menu).
As for the bluetooth, do you want to use it for file transfer, or are you trying to connect headphones and/or a keyboard?