Backup drive unreconizable, but my other passport is, why?

[Melissa505]

I have two backup drives. Both are WD and My Passport is the one that is recognized. The other one hums and vibrates saying its working or running, but I can't find it anywhere on my computer.

 

[Phillip Corcoran]

It may just need a drive letter assigning to it, in which case it should be shown in Disk Management. Without a drive letter Windows Explorer cannot show it. See this guide on how to add a drive letter to it:
http://windows.microsoft.com/en-gb/windows/change-add-remove-drive-letter#1TC=windows-7

In Disk Management the drive, if it shows up there at all, will have no name or drive letter and will have "Raw" as it's volume description.