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Its possible that your user Permissions on the external drive/s got changed with the OS update.
Right click on the drive and select get info and ensure you have Read and Write privileges for the drive.
Thank you - Im going to back up the drive first then will try and change the permissions
1 person found this helpful
If the external drive is formatted for NTFS, it may be that your third-party driver (required for writable NTFS drives) has expired. See this thread: Re: Lr cc Classic, can not be opened on network volumes, removable storage
Thank you - that solved my problem.