It may simply not be Partitioned or Formatted in a manner your OS can recognize. IF there is nothing on it you want to save, here's how to wipe it clean and start fresh.
You'll use Windows' built-in tool Disk Management. Click on Start at lower left, then RIGHT-click on "My Computer" and choose "Manage" on the mini-menu. In the resulting window expand "Storage" if necessary and click on Disk Management.
Look at the right-hand portion of this window. It has upper and lower sections. Each SCROLLS to show you all it has. The upper one shows you all the drives Windows can use right now. The LOWER RIGHT window shows you all the valid hardware devices, including some not yet understood. Look there for your external drive. In this pane, each device is represented by a horizontal block. In a block the left end shows a system name like "DISK 0", a type like "Basic", a size and bit more. To the right the major block may be one, or may be subdivided. In each of these sub-blocks there is a letter name like "C:" assigned to the "drive", a size, a File System type like "NTFS", and a status. You will see all of those for your C: drive. Now look at others below it. An optical drive will not have anything in the right-hand major block because there are not adjustments to be made. But your external unit should be in the Lower Right pane as one horizontal block with a left label block plus a large white right-hand block with NO letter name. If it is there with size and other stuff in the big white block but just no letter assigned, RIGHT-click on that big block. A mini-menu will offer to let you assign a name (letter), and you can give it any letter not already in use. If you do, back out of Disk Management and reboot your computer so Windows can adjust to the new info. Your external drive should show up now.
On the other hand, that big right-hand white block may say the disk space is "Unallocated". If it is, that means you need to do two steps. (Well, many versions now combine these into one for easy use.) First you Create a Partition, then you Format it. A Partition is just a chunk of the HDD unit's space defined as one usable piece called a "drive" with its own letter name and used as a separate "drive". You can create a few Partitions on one HDD, or just one that contains ALL of the HDD's space. I'm betting you want one big Partition. RIGHT-click on the Unallocated Space and from the menu choose to Create a Partition. Choose options to make it a Primary Partition, but NOT a bootable one (you are using this only for data storage, right?). Set its size to the entire drive space. If you are in a Wizard and see options for Format type, File System, etc, set them now. Choose the NTFS File System unless you know you need the older FAT32. I recommend you choose to do a Full Format, not Quick, on an older drive. A Quick Format will create the File System and get the unit ready to use, taking about 15 minutes. A Full Format will do that and then go through the entire HDD and test every sector, marking off any that fail so they are never used again by Windows. This can take several hours so be patient! But it's worth the time to check the old unit thoroughly before using it.
If your version of this tool just offers the Partitioning options, set them and run that step first. After the Partition is created, RIGHT-click again on that new Partition and choose to Format it as a separate second step, setting options as above, then let that run. When both are finished, back out and reboot. Your external unit should now show up in My Computer with the letter name you gave it, ready to use.
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