"The Steam client will not allow more than one library folder from the same drive. This process cannot work around that restriction.

Go to (1) Steam > (2) Settings > (3) Storage
(4) Click on the (+) symbol on the far right to add a new Steam Library Folder. { * }
(5) Click on the list of drives to open the menu of available locations.
(6) Select "Let me choose another location" at the bottom of the list.
(7) Click on the blue Add button.
(8) A Windows File Explorer window will open. Select the drive and folder that you want to use. Do not choose the \steamapps\ folder or any folder inside of it. You want the main Library folder.
(9) Click on Select Folder.

The new drive will be listed among the available Steam Library folders.

This may seem convoluted, but most users will never need to know this.

Linux and Mac should have the same overall process but with whatever file manager window those OSs use.

Recent change:
{ * } Seems this has changed slightly. It is now a drop-down menu that contains your library drives. After clicking on it to expand it, at the bottom is a + Add Steam Library selection which will take you back to Step 5 above."