I have owned a My Passport for Mac 4TB for four months (January 2021). I have it partitioned. One section (3TB) holds about 16,000 pictures and the other is for Time Machine backups (1TB). Neither section is even close to being full. Multiple times a day I will get a message saying that Time Machine was not able to complete a backup. When this appears the only solution is to restart the MBP. Related to this is the fact that the screen saver will display no photos because it cannot access the photos in the external drive. Again the solution is to restart the MBP.
MacBook Pro (15-inch, 2017)
macOS Big Sur version 11.2.3 (20D91)
WD Discovery v.4.3.327
