When you import images into the Lightroom Catalog, Lightroom keeps track of where those images live — in what folder, on what hard drive. This shows in the Folders panel in the Library Module.
Imagine moving the car while your family or friends are inside the convenience store. (I confess to having done this once or twice!) When they walk out, you can imagine that they will be confused, since they weren’t around to see it happen. It is the same with Lightroom: if you move your files and folders while Lightroom isn’t looking, it loses track of them and gets confused (displaying the question mark.) If you use Mac Finder, Windows Explorer or any other utility to move your files or folders, Lightroom can’t see that you have moved them.
The solution is simple: do any moving of files and folders within Lightroom. In the Folders panel you can click on a folder and drag it inside of another folder, on any hard drive that Lightroom is aware of. This physically moves your folders and images just like using Finder or Explorer would, but Lightroom is able to keep up with you. If you want to move selected images from one folder to another, click on the folder they currently are in so that they show up in Grid (thumbnail) view. Click on the thumbnail and drag the image to another folder in the Folders panel.
If you do get a question mark on a folder, right-click (ctl-click on a one button Mac mouse) on it and choose “Find Missing Folder”. Point to the folder in its new location, click choose, and the question mark will go away. If you get a question mark on an individual image, click on the question mark next to the thumbnail, and choose “Locate Missing File”. Point to the file in its new location.
At least Lightroom doesn’t get angry, like friends or family sometimes do!