The problem is caused by this:
If you have installed Steam on your D:\ driver, and you have the old Steam Library on your D:\ driver before, you will find Steam won’t let you add this old Library folder because Steam only allows one Library on the same driver. For D:\ driver, at present, it has been the default driver for Steam Library while you are installing the Steam main program. You can’t even change the default folder to another folder on the D:\ driver, it has been fixed by Steam.
There are two solutions:
Solution A:
Keep the old Steam Library on D:\ driver and then uninstall and reinstall the Steam on C:\ driver. Open Steam Settings, find the Downloads tab, click the STEAM LIBRARY FOLDERS button, then add the old Steam Library folder on D:\ driver.
Solution B:
To make sure of success, please quit steam.exe first.
Open the path D:\Program Files\Steam\steamapps (assume you installed Steam on D:\Program Files\Steam), this is the default Library folder after installing Steam. You can see an empty folder named sourcemods and a file named libraryfolders.vdf. Delete these two items. (You can backup these two files in case you need them later)
Then go to your old Steam Library folder, you should see a folder named steamapps, a file named steam.dll and a file named libraryfolders.vdf. Open steamapps, Cut (Ctrl + X) all the items (Ctrl + A) in this folder, and paste (Ctrl + V) into D:\Program Files\Steam\steamapps.
Now you can launch your steam.exe and see if the Library merging works.
Jaylin Wu 