Once in a while, I reinstall OSX. I have both a SSD for the system and a SATA drive for
/Users. This way, I can recover my personal files in only a few seconds. The problem is, when I do that, the Aperture library disappears from the desktop background selection.
I found a tweak on the Internet which states that you have to go to Aperture’s preferences, in the “Previews” section and unset/reset “Share previews with iLife and iWork”, restarting Aperture between switch. That didn’t solve my problem.
What worked was selecting the “Photos” view from the “Library” section and, in the “Photos” menu, “Delete Preview for Library”. Then you seem to be able to let Aperture recreates the thumbnails ; I choose to “Generate Thumbnails”. After a while, the pictures were back in the background selection.