Hello TehDeathtrap. I would like to know if you have found out more about this. I have exactly the same drive (2.5" Sata). I had id for almost a year, and exactly the same just happened to me 3 or 4 days ago. Based on when it happened to you, It could be something related to the firmware version. I have installed the WD SSD utility but unfortunately there is no update that “could” fix the issue. I was planning on formatting my drive and see if that helped, but it seems that based on your experience it will not. Also, I tried different SATA cables, different PSU cables as well as different Windows installations, and the same happens.
Also, in my case after maybe 5-10 minutes the activity drops down to 0%, and the SSD works as normal, but after maybe an hour or two the same happens again. Also, I noticed that always happens at boot, and if you hot swap the SSD, and you plug it back, it works as normal dropping to 0% activity.
Hope that we get an answer soon, in case that you still have the issue as well.