Because they report two different things.
If this is a new hd, perhaps its connected to a controler that requires
third party drivers?
Other than that you dont post enough info on problem, eg how/when
Device Manager should see the disk drive through its ID
chip or bios in its electronics card or circuit via the
connection through the ribbon cable. Disk Manager should
also see the disk drive unless it is unpowered and the HD
platter is not spinning. Check the power connection to the
HD.
