I have a 4TB Seagate Backup Plus drive and recently bought another one to back it up. The drive that arrived (another Backup Plus) was exceedingly slow during data transfers; it would show copy times for a 2 something TB folder in days and then stall. I reckoned that I'd gotten a faulty drive and asked for a replacement, which behaved the same way. I then replaced it with a WD My Book 4TB. All the data copied over ok but I've noticed that the WD has taken a huge hit in speed after the data transfer. A Blackmagic Disk Speed Test comparison showed the read and write speeds go from around 150 MB/s when the drive was empty to around 70 MB/s when I had transferred everything over (225 GB of free space was remaining at this point). Surprisingly, the original Seagate drive (the one that works fine), which has the exact same data on it, doesn't show such a drop in speed and clocks in at around 120 MB/s for both the read and write speeds. My questions are: 1 - Is such a drastic drop in speed (from 150 MB/s to 70 MB/s) normal when a drive fills up or could the WD drive be defective and I should ask for a replacement? 2 - Could I be doing something wrong while formatting the drives? I use Disk Utility to do a simple Mac OS Extended (Journaled) format and don't use the WD turbo or other such drivers. I've been doing this for years and am only asking as I am surprised at the number of drives I've faced issues with this time.