It happened to me to connect two identical HD (same brand/model and size) to my Windows PC in order to make a copy of files from one to the other. However only the first connected to an USB port was always seen!!

After a some research on Internet I realize that this was caused by a Signature Collision of the two drives having identical signatures. In general every Hard Drive is given a unique signature in the partition sector of the drive: however, in rare cases, two HDs can have the same signature (e.g. one is cloned using a third-party backup SW).

So the procedure to make both be accessed from Windows is the following:

1. Right click the Start Screen icon in the lower-left corner of the screen and click Disk Management.
2. On the lower part of the Disk Management window, look for a disk which is listed as Offline.
3. Verify that you have a Signature Collision by holding your mouse over the drive. You will receive a pop-up explaining why the drive is offline
4. Right click on the disk in the area that says Offline, and select Online
5. The drive will be given a new signature. A new Drive Letter should appear and you should now be able to access the drive.