I believe that WD Password uses faulty SATA to USB 3.0 boards. The board starts failing at USB 3.0 and down grades the negotiations to a very slow USB speed. Perhaps 1 or 1.5 . That is why the transfer is slow.
I am working on one right now that takes 20 min just to some online to copy the data. I am copying data and it is super slow. But it is copying just be patient, get the data off first then trash it and buy a new one. I should take days even weeks. I use the (sudo find /path_to_source | cpio -pdmv /path_to_destination)
That way if it fails from a power outage OSLT you just run the command again and it skips existing files