If your friend lived at 100 Any Street, and you had that address in your contact list, then he/she moved without telling you- would you know where he/she now lived?
Likewise- The Lightroom database that has been given an address for your photos is 'lost' if you move their address without telling Lightroom.
A change of Disk name is a 'change of address' , so you have to re-link the catalog folders to their new address.
This can be done easily by right-clicking a folder in the Folder panel of Lr and re-link it to the new location (ie. the new drive name)
Read the following link -especially the topics about missing folders-
You can follow @wobertc's advice ... or you can "un-rename" the drive to fix the problem; and rename the backup drive so you can tell the difference.
So the bottom line is you should not of changed the original drive name and changed the second, new, drive that you added as a backup drive.
If you want LR to know where your images are don't ever change the name, OS X, or letter, Windows, the drive was using when you imported your images. Otherwise you will always have this problem and always have to use either Update folder location or the Find missing files systems in LR to reconnect the catalog references to the actual images.