If you use referenced masters in Aperture 3 you are probably going to want to know about this. Especially if you use them on multiple volumes. Somewhere along the way, maybe with the initial release of Aperture 3, maybe later, I cannot be sure. Aperture stopped working the way it used to in Aperture 2 with respect to changing out a volume of referenced masters with a backup volume.
Say you have your Aperture library on volume "V1" and a second drive with all your masters called "V2". You also have a backup of "V2" called "V2backup". In the past if your "V2" drive went bad all you needed to do was hookup "V2backup", rename the volume to "V2", fire up Aperture and off you went. Done. With Aperture 3 you also need to use "locate masters" to reconnect all the masters on the drive. If you don't they will continue to be disconnected.
This is not a huge deal because the "locate masters" in Aperture 3 seems to work a little more reliably than it did in Aperture 2 but all the same it is not as convenient as it was and a potential pit fall when recovering from an drive failure. I discovered this a few weeks ago when attempting to help out a fellow Aperture user with drive issues. I then made sure that the behavior was the same on my systems since this is something that I had not tested yet.
The funny thing is I cannot tell if this was an accident or an intentional change. I cannot imagine why Apple would think that anyone would want it to work this way. The reason this happens is that Aperture stores the volume UUID (virtually impossible to change by normal people) along with the volume name and path of the referenced files. The way that it seemed to work in the past was that Aperture used the volume name if it existed and the volume uuid if the volume name changed - this way Aperture would help you out if you renamed the volume via the volume uuid AND it would work by renaming a backup to the same name as the original volume. In Aperture 3 the UUID must match or Aperture will not recognize the volume as the same even if the paths, file names, and file attributes are all identical with out the explicit reconnect via "locate masters" on your part.
If you do not like this behavior I implore you to submit feedback to Apple.