I use latest Mac operating system on a 6TB My Book for Mac. About once a week, I get a message that the Time Machine Backup failed. Sometimes a similar problem is evident when I use Adobe Photoshop Lightroom and the program does not recognize all files.
This is a fairly new 6TB. I’ve had similar problems with another 6TB. My fix is to eject the 6TB (normally requires a force eject) and then to go to disk utilities and mount the drive. Then it works fine. I have run first aid in disk utilities and the report is that all is fine.
I have to assume there is some sort of problem with the WD 6TB’s working with Mac’s and that it is an intermittent problem. My work-around is relatively easy but is annoying. I don’t believe I had this issue in the past with 4TB drives and smaller.