Problems with WD-Hard disc in my MC EX2 - 2HD's

Hello I have some trouble with the new installed HD. I use a MC EX2 with 2 HD’d in a Raid system.

Some month ago the original delivered 2 years old HD became the indication “Bad” with red LED in Front.
Therefore I have replaced both HD’s by the roiginal type of HD used by the manufacturer.
Now afetr two month the first HD became the red indication again, so I have replaced this by a new one. Now afeter 3 month the second HD have the same indication.

Does any body have an idea, why the first HD-set works for two years and the new on only for three month!?