Aperture may need "rebuilding" but is stuck in "repair" mode

I'm trying to open an aperture library that is stored on an external hard drive. Initially, I got a message saying something to the effect that repeated forced closures could indicate the library needs repairing, and then I was given three options for addressing the issue. I forget what the first option was, but the second (and default) option was to repair the library, with the third option being to rebuild the library (the suggestion being to choose this if the first two options didn't work). So I selected the default option to repair the library. After an minute or so, another message popped up saying that the library would need "rebuilding". However, there is now a progress bar on the screen which says "repair library: examining library", except that no progress has yet been indicated. However, it doesn't seem possible to quit Aperture, as the "Quit" option is greyed out.

I don't know if I just need to be patient and wait, or to force close Aperture, and then re-open and select "rebuild" when the options appear again.

Any advice?

- David