I had a weird problem with my MacBook Pro and USB hard drive. I tried to copy large files but every 15 minutes or so the hard drive disconnected and after a few seconds connected back. Obviously that interrupted the copying operation which was pretty annoying.
I found out that the USB ports are not powered equally. In my MacBook Pro the problem disappeared by switching to the port nearer the screen. After that the hard drive worked reliably.
It seems other laptops have similar problems, so if you encounter problems with not having enough power or unreliable connection, try another USB port.