Last Sunday I decided to finally bite the bullet and install Aperture 3. Now the way I had it setup was that is was on an external Fat32 formated drive so we could share the library between any accounts we have on our Mac
So after gleefully upgrading to Aperture 3 I discovered that
1) My existing linbrary on my Fat 32 drive would not upgrage because the filesystem is unsuported by Aperture.
2) Aperture somehow got it into its head to delete my entire library because of this. Seriously 95Gb of Photos magically dissipated in about 1 minute thanks to the bloody program.
3) Even if you do manage to upgrade a recently backed up copy, OSX's permisioning will cause untold havoc with your library if you try to share it with multiple users on the same PC. Thumbnails don't appear. Faces aren't recognized. Aperture crashes. Etc etc...
This is how I got around the pesky problem.
1) Make sure you have a backup and don't let Aperture anywhere near it!!
2) Reformat your external drive to Mac OS (Journaled)
3) Set the ACL permissions to read Write for Everyone
4) Do a Get Info on the drive and set the 'Ignore Ownerships on this volume'
5) Copy your aperture library to the drive.
6) In any account that will share the library, log in and again do a Get Info on the drive and set the 'Ignore Ownerships on this volume' as this seems to be account specific. Who knows why.
On my system its all working OK. iTunes is synching the photos to my phone and any others and we can all edit the photos in the sahred library.