Had similar problems with external USB hard drives and found a combination of the following would SOMETIMES help:
1) Connect the hard drive to a port on the machine iteslf, not to a hub (even a powered hub). Best case, a port on the back of the case. Sometimes, hubs (and even front ports) don't provide enough power for hard drives.
2) Check your USB settings, and once you've confirmed that the ports to which you're connecting the drives are all USB 2.0, change the BIOS setting to disable USB legacy support.