Can I leave the external disk pluged in at all times

I have my Hitachi 2,5 disk pluged  in the machine at all times and I can acces it over the network even if the WD TV is turned off. Is that bad for my disk, I mean do you unplug it if you are not using it or you just leave it there. will I kill my disk or an USB key that way.

Sorry if this is a noob question and thanks for the answer.

Or do YOU leave your external drive pluged in the WD TV LIVE or do you remove it?