Import function has Add grayed out

I have copied my files from a card reader to my Mac's hard drive. I am now trying to "Add" these pictures to my library. However, the Add option is grayed out. The only thing that I have done differently is that I imported a file directly from a card reader earlier in the day, and hence had to use the "Copy" option for that file.

The workflow described above, ie copy files to their destination manually (outside of LR) and then use Add is something I have done for years. Why had is now suddenly stopped working.

Any help is appreciated.

Regards

Bob.