Recently I had to reformat my Drobo, which is where I keep the Master Files of my main photo library. After moving everything off the Drobo to other drives, reformatting the Drobo volume, and moving everything back to it, I ran into a problem with one of my recent Aperture projects: it would not recognize the new location of the Master Files. All the images for that project (about 1,200 images) were flagged as offline. All the other images in other projects reconnected without a problem, but this one specific project would not reconnect to the master images.
I tried using the Locate Referenced Files (File>Locate Referenced Files…) command and that's when the real trouble started. I navigated to the location of the master files on my newly formatted Drobo, but despite recognizing the image, the Reconnect and Reconnect All buttons remained greyed out and unavailable.
After a fair bit of research and investigation into this issue, I discovered that you can essentially force the Reconnect function by holding down the Option + Command keys while you are in the Locate Referenced Files window. This will make the two reconnect buttons active and allow you to reconnect to your master file, wherever it exists.
After finishing the master file reconnections, I still can't say for sure what caused it. I suspect the Drobo volume, newly formatted but with the same name, may have caused some some issues, but this is where I sidle up to the border of my knowledge of drive structure and OS file naming systems. Either way, it's a handy trick when dealing with multiple drives and volumes.