Check in Finder if the folder exists in this location. Then - still in Finder - check if it contains any photos.
a) If it doesn't contain photos, delete it and try to move the folder in Lr again;
b) if it contains some photos that don't show in Lr, import the photos in Lr. This will display the folder in Lr. Then in Lr move the photos (not the folder!) from the old location to the new one.
Thanks for the suggestion. I tried this. The folders do exist in the new location, but they are empty. I deleted the empty folders in Finder, then went back to LR 4 and tried to import the folders into the new drive. This time LR gave me an "Unknown error" message, which is terminally unhelpful. Same thing ("Unknown error") when I tried to just grab the images and import them into a folder on the new drive.
Don't import a folder that's already in Lr.
Importing should be used only for images that have never been imported.
Just move the folder in Lr. That means you drag-and-drop the folder in the folders panel.