switch to iPhoto library option no longer shows
I'm using Aperture 3.5.1 on Mavericks 10.9.2, with iPhoto 9.5.1.
Both have been working fine together, until now - when suddenly the option to switch to my iPhoto library has disappeared from Aperture.
Edit > Switch to Library now only shows Other/new, where previously iPhoto was the first option (appearing separately, above).
The set up is on an external drive, with the iPhoto library on a partition. I have a clone of this external drive, for backups. The clone has not been updated since the problem appeared, so it should not have the same problem. However, when I booted from the clone, and launched Aperture, the same thing now shows: the switch to iPhoto option is gone.
I've tried deleting the preference file, but it made no difference.
What could have suddenly happened? And any ideas how to fix it?
OS X Mavericks (10.9.1)