Try a Right-Click "Remove Folder" on the ?F:lightroom folder shown in your screen-pic.
Then try to remove the F: drive entry.
Maybe LR will not remove the drive if it thinks you still have a folder in the catalog.
Thanks, but I indeed tried that with both the root (F) and the subfolder (lightroom) items. I thought I mentioned that. The menu item is there, it just does not do anything at all. No error message, kust nothing happens.
Try exiting LR, restarting your computer, and then restarting LR. In other situations, that has caused LR to forget about the old drive letter.
That did it, thanks!