If you have individual folders in the Library with question marks try right-clicking (control-click) and choosing "Update Folder Location"
You may need to do this individually if it doesn't work on the root folder and nested sub-folders.
There is another issue going on here as LR does not care what you do with old backup files. (Most folks just delete them.)
Hi, 99jon, that link was super helpful. EXCEPT... when I locate the new location and highlight the right folder, the folder appears empty (although I can see in Windows Explorer that it's not), and nothing happens when I select it. The ? remains in LR and sometimes the computer freezes up, so I'm getting nowhere.