I have a 3 TB MyBook for Mac running on my 2007 Santa Rosa MacBook Pro and it constantly ejects when trying to copy backups to it. I have it partitioned into 3 1 TB sections with 1 being a Time Machine backup of my MBP and another being a Carbon Copy Cloner backup of another external WD drive. Whenever a large backup is running it will only last 15-20 minutes, then all partitions will eject themselves. Even if I leave the drives connected without any backup tasks running, it will eventually eject itself.
I’m about to try uninstalling the WD Drive Utilities and WD Security applications to see if this will help, but thought I would see if anyone else is having the same problem.