If this is a new external drive and you have not yet Created a Partition on it and formatted it, that is why. Any new empty HDD needs these two operations done before the hardware (Device Manager sees hardware) can be used by the Operating system. My Computer can only "see" data storage devices that have been given a File System in a Format operation so it is ready to use.
Read up on Partitions and Format, or perhaps under the heading of "Initialize a Disk". These can be done in windows Disk Management.