I purchased a 16TB EX4 system in February of 2014. About 11 months later I had a drive failure. I called WD and they shipped out a replacement drive and all was well. Now in February of 2016 the same drive (#1 slot) has failed again. When I took out the failed drive much to my surprise I saw stamped on the side of the drive that it was a re-certified drive. I’m guessing when the new drive arrives it will be a re-certified drive also.
I haven’t experience any issues with a re-certified unit since they are tested before they ship out.
When you get the replacement, make sure to test the unit as well to confirm that is working at 100%.
The re-certified drive tested just fine before I started to use it, it failed about a year later.