Completely killed my iMac ..unable to restore back to any version as it cannot find any backup in timeline. Apple Support were useless (over the phone) and were unable to provide any additional support other than "you will need to take to a store" Having to take to a apple certified repairer to reinstall OS back to a suitable OS ...just hope this doesn't cost the earth. Seems that the backups dont like the new file OS file system. This will be the last beta upgrade I participate in ...fell let down by Apple not testing the release update, its not as if I have upgraded to R5 ..it was a standard system update !!