Does anyone know if it's possible to Flag one image when in the compare mode? I've tried toggling the “Primary” button at the bottom of my screen but neither on or off makes any difference. What I'm hoping to do is look at a collection of images and put a flag on the ones I want to keep track of. However, when they are all showing in the compare screen, flagging one image puts a flag on all of them. Any suggestions?
To flag the primary selection in a group of selected images you can click the the control at the top of the Info inspector. Using the menu item or keyboard shortcut will flag all the selected images, regardless of the Primary Only setting.
Note that if you use ratings or labels rather than flags, the Primary Only setting applies.
Also, based on your description, it sounds like you aren’t really using using Aperture’s Compare mode, but rather selecting a group of images and showing them all in the viewer. It’s not quite the same.