Are they both IDE hard drives? If so, did you set the jumpers correctly? If you know you set all the jumpers correctly and everything, go to start>control panel>Administrative tools>Computer Management>Storage and see if the hard drive is shown there. If it is, it means it has not yet been formatted and you must format it before you can use it. However, if it doesn't appear here, that means either the jumpers are improperly configured or the hard drive isn't plugged in properly.