bsprague wrote:
If Bruce's approach doesn't work, try the "+" in the upper right of the folders box and see if it lets you add a drive. If I remember right, Lightroom does not try to show all the drives on a computer, just the ones you tell it has image files. If that doesn't work, put an image file on the new drive and try "Importing" it. It will make you pick a source drive and that should add it to the list.
Actually, the Folders panel in LR shows folders that have been imported (cataloged) by LR. LR has no idea what you have done while it is sleeping. Import or have LR move some folders the and it will appear on your screen. As many have sadly found Finder/Explorer view is not necessarily the same as LR’s view. Good to remember.