john, when you say replacement hard drive, why did it need to be replaced? The one you got is a good drive from a reputable source? Any chance you could try the drive on another computer? Check all connectors to be certain that they are properly cabled and firmly seated. make sure that your cable is not reversed. Make sure that you matched the colored strip on the cable with pin 1 on the drive. Let us know if that made a difference and if all connections are the way they are supposed to be. The [http://www.coyotetale.net/1sand0s/dellgx260/index.htm|html Service Manual] is available for you. Good Luck