FOUND A SOLUTION
At least, it was a solution for me.
I was having this same problem, didn’t happen until I updated to El Capitan
I called apple, and had GREAT support from them, and we found the solution to my problem
There is “wd drive utility” software that would launch upon start up
go here to delete it:
FINDER - GO - COMPUTER - LIBRARY - LaunchDaemons - com.wdc.WDPrivilegedHelper.plist
DELETE THAT FILE
Then, go to
SYSTEM PREFERENCES - USERS & GROUPS - LOGIN ITEMS - UNCHECK WD DRIVE UTILITY
That fixed the porblem for me, had the issue on 2 of my 4TB WD MY BOOKS
So glad that apple helped me solve the problem, props to them! Hope this helps you guys!