These are my external drives (I have both the Mercury Elite Pro size and the newer Mini version - two of each):
https://eshop.macsales.com/shop/external-storage?_ga=2.239729731.117918172.14960 12229-1347876559.1495985158
They actually had a problem with the Oxford chipset in some of them some time ago and my iMac would not wake up or actually shut down if one was attached (and the drives would stop working as well). Macsales worked with Apple whose engineers acknowledged that there was some sort of change during an OS update which made the chipsets of the external enclosures not be able to communicate properly with the iMac. OWC switched out the drives for me (they were covered by warranty) and the new ones had different chipsets in them. I do not know what chipsets WD uses and would not be able to look it up without knowing the exact model/year. Nor would I be able to guess - it is a tricky problem and of course, everyone will blame the "other guy". This is the chipset they are using now in the Pro Mini:
I did find this article which shows far more specs than the WD website and does mention an Oxford chipset for some of their drives.
The only way I can think of that you could prove that it was one or the other is to have a different Mac with a different (older) OS version and test one of your drives with that. If it does the same thing, then it is not OS related (obviously).