..and your backup Catalogs?
I'm not clear whether it's your standard catalog or the backup that you refer to that your're having trouble with?
Either way, what about your other backups?
If six months worth of work is missing then that's most likely because it's an older file. Presumably a six month old backup?
When i open lightroom, prior to installing and updating to yosemite, it would open the catalog stored on the external drive.
the date on that file is 10-17-2014, thus current.
my lightroom catalog is stored on the external drive, same place where all my pictures are.
as for backups, they are identical, same issue.
i dont understand why lightroom when opening my catalog on 10-17 was fine but when i update to yosemite and open exact same catalog its missing 6 months of information, despite the fact date on catalog is 10-17 and is same file on same location on an external drive.
i tried a backup file and same thing.
how are you determining 6 months? do you see history on your images in the Develop module? perhaps you need to rebuild previews
You probably need to use your operating system to search your external drive for all .LRCAT files, and open each one to fidn the one that has the current set of photos