Is it a new drive that you haven't formatted yet? If it isn't formatted, it won't be in my computer because there isn't a partition to mount yet. Look in Disk Management and see if you see it there. If the colored bar on that drive in disk management is black and the drive says raw or unallocated, then the drive isn't formatted and you'll need to format it.
EDIT: Yea, what Traildriver said. My bad, you responded while I was typing.