I am no expert, but could it be that the TLER utility does not work correctly on the WD10EACS? I mean TLER was an issue a while back, but I don't have information about the current gen drives. Have you tried reverting the drives?
In the end, maybe using that util broke them?
I somehow doubt using the util broke the drives two days later. I suspect it had something to do with a non-stop read-write load. As for reverting the drives, they won't even spin up when connected to my desktop anymore.
Manufacturing dates are 25 and 30 December, Thailand.