Macs OS 10.12.6 but have had same issue on previous OS versions. Using TimeMachine.
We’ve been getting this message off and one for years. I usually have to turn OFF the drive and restart it to get it to work again. Same thing happens from other computers as well. Although when one computer gets this message, the other computers can still backup to it, so it seems to be a file issue of being in use, and not the drive.
It appears files aren’t getting released when it finishes backing up, so the next time it tries the files look like they are still open.
It seems that a file which is being used by the Operating System is causing this issue. It’s best contacting OS manufacturer for resolving this issue. You can run diagnostics on the drive to check health status.
The file that is ‘already’ open is the backup on the WD, this has persisted thru several different Macs and OSs. NEVER have a problem on hard drives connected to computer. I don’t believe the WD is ‘closing’ and releasing the file properly after it gets to a certain size.
