I purchased Five WD 4TB Passport drives to back up a WD MyCloudPR4100 24TB (w/ Raid 5) NAS. Unfortunately, the 4TB Passports seem to have a write rate problem. I use a Robocopy batch file with 8 threads running on a windows 10 pro (i5 with 16GB RAM) desktop to back up the MyCloud. I am using a USB3 interface. Backup starts just fine and for the first 2TB, write rates are 80-100 MB/s with an average response time of ~1500 ms. After that, write rates fall precipitously and ultimately drop to 2-3 MB/s with an average response time of 4500-5500 ms by the time the drive has 2.5 TB transferred. After that, the write rate seems to be stuck at a low rate. Disk shows mostly 100% active time.Things I have tried:1. Quick format the drive: Write rate remains stuck at 2-3 MB/s2. Full drive format (takes 12 hours): Write rate is again good for the first 2TB.3. Halt backup script and just let the drive stay powered up for 8 hours: Partial write rate recovery for a very short time.4. Switching to performance write (enables cache): no effect as compared to cache disabled.5. Checking fragmentation: windows says the drive is not fragmented.6. Altering the back up script to write large files first (PLEX .mkv .mp4 mp3) and then the user files: No difference from writing small user files first.7. Disable password protection: no difference.8. Backup directly on the MyCloudPR4100 by defining a backup plan on the MyCloud: still degraded write rate.9. Used a powered USB3 Hub in case the drive was getting power starved: no difference10. Disable Anti-virus SW: no difference11. Run CHKDSK: no errors found12. Change default allocation unit to from 4k to 32k: No differenceThere is no way to disable windows write caching on the WD Passport.I have already returned one drive to Amazon, but the replacement drive has the same characteristics. In fact, all of my 4TB drives have the same issue.It is interesting to note that I have a 3.5 inch 3T WD MyBook and a 3.5 inch 3TB Seagate Backup Plus both with external power and usb3 and neither seem to have the problem.Since write rate does not reset to near 100MB/s unless a full format is done on the drive (quick format does not help at all), and the problem has duplicated itself on 5 of 5 drives that I purchased, I suspect there is something fundamentally wrong with the 4TB Drive.WD, Fix your drives!March 11, 2018I edited this review on Feb 26, 2018, but the update was not posted. I have reposted my update.UPDATE Feb 26, 2018I have completed a much further examination of the 5 4TB Passport Drive I still have.WD suggested using their disk tools to validate the drives are good. All 5 drives passed all tests when using the WD Data Lifeguard tool. When I attempted to erase the drives with the tool, the erase failed.I then used the WD drive utilities to rerun diagnostics (all passed) and then the tool was used to erase the drives. From the messages that pop up on WD, it appears that the erase/wipe tool deletes the partition and then creates and formats a new partition.After using the WD drive utilities to wipe the drives, I reran my drive performance check which was composed of transferring 2.7 TB (185k files)from my MyCloudPR4100 via my 1Gb/s LAN to a Dell Latitude 3470 laptop with the drive plugged into a USB 3 input. The transfer was done using a .bat robocopy with 16 threads and no directory or file listing. Files were mkv, mp3, and misc (mostly jpg). The laptop had no other activity running. The WD MyCloudPR4100 was connected to a Netgear ProSAFE GS108 gigabit switch which was also connected to the Dell Latitude 3470.Drives were set to use the default profile (safe removal, no caching) and indexing of file contents was disabled.The windows performance monitor was used to log the write performance of the drive each minute.All 5 drives were tested and the results are summarized in the attached .jpg. Initially, write performance on the mkv files was approximately 70GB/s and degraded slowly as seen in the attached graphic. mp3 and jpg write performance was approximately 43 MB/s.As a further test, I erased the 522GB of the "mostly jpg" workload and then proceeded to write 924GB of mp4 files to the drive. Write performance was 27-34 MB/s.I should note, the MyCloudPR4100 is used as a Plex server and is also used as DVR storage and backup for PCs on the network, so some occasional performance degradations could be attributed to other network or server traffic.So in conclusion, I have upgraded my rating from a 1 to a 4. WD did not get the 5 rating because of my out of the box experience noted above on all 5 drives. The is no reason I should need to use the WD tools prior to using each drive. They should work well from the factory.