I just bought a new 4 TB My Book drive, and using the supplied backup software, used it to back up one of my internal drives that contained 904 GB of data. When complete, the new backup on the My Book drive contained 2.67 TB of data!
What gives? I thought the backup data sets were compressed and would take less space than the original drive… now I find the backup takes up 3 times as much space as the source drive! I could manually copy over my files for a 1:1 space usage.