When her old Mac Mini started acting up when connecting multiple hard drives, Denise was encouraged by an Apple Genius to buy a new one. So she did and now she'd like to set it up properly with Time Machine, external hard drives, the works. Leo says buying a new computer to solve an external hard drive connection wasn't really the right solution. Connecting all drives into a common USB bus would make any drive unreliable. It's far better to have drives with their own dedicated power. That makes them more consistent.
Denise also has problems with her USB keyboard. Leo says that may mean that the USB hub is overwhelmed and is becoming unreliable. He recommends unplugging all the external drives and then connect them one at a time, testing for reliability. When they start being unreliable, then she knows how many the bus can handle.