Is there a limit to the amount of external Hard Drives with a (New) Mac Mini

Ha all...

I have a new Mac Mini (Quad i7 16 GB Ram) and I'm having a problem with plugging in a USB 2.0 external Hard Drive. It basically just doesn't appear anywhere on the screen. I have two 3TB external drives which seem to work fine but for some reason I can't seem to get the 3rd Hard Drive on the screen.

Is there a limit on the Mini as to how many external drives or total disk space which can be used? Maybe I have a dead drive but I wanted to find out first if there is any expansion limitations for HDs.

Thanks In Advance...