Power on all the time?

I’m curious about something.  I’m using my My Book drive on an older desktop PC with only one USB port,  I bought a powered hub with 4 ports on it, but I realized that when connected to the hub the external drive never powers down when the PC does.  Can anyone tell me if I need to worry about shortening the lifespan of the drive if I continue to use it this way?

The My Book should go into sleep mode automatically when is not being used. Do you still feel the My Book internal disk vibrating after long periods of not being used? 

Thanks, Ichigo.  No, the drive does not ever go to sleep when using the powered hub.  It will go to sleep if I just plug it in the slot on the tower.  The problem is that all the slots are then in use, and there is no place to plug in a flash drive or MP3 player.  That’s why I bought the powered hub–it adds 4 USB slots.  That’s why I asked the original question:  can I leave power to the external drive all the time, or will it be damaged?