It's been several years, but I do know that I used WD green drives in an old Drobo 8 bay NAS, and they did NOT work well.
I replaced all of the greens with wd black drives, at the time I think they were considered enterprise desktop drives, and they worked great.
Those were eventually replaced with WD reds, which also worked great. The black drives that were in that array, are ALL still functioning in multiple workstations and are approaching 6 years old. They were used in the NAS for about 2.5 years non-stop
I realize that a Drobo isn't a freenas box, but they do have general functionality in common, so I would like to hear from those who are using desktop based drives, and get some feedback from them.