I have a similar problem on my Terastation (HS-DH0.0TGL/R5).
Raid Error E13 Array 1 Error
Operational Mode I12 Degrade Mode
HD1 Error E23 HD1 is faulty
Several months ago, I replaced a drive in this Terastation and everything went well. At that time, I purchased two new drives, keeping one as a spare. These drives were IDENTICAL to the drives installed on the Terastation (Samsung Model HD501LJ, 500Gb).
I replaced HD1 with the remaining new drive I had, but am still getting the same messages. I've done some reseach on other users with the same experience and many are now saying that Buffalo Technologies is suggesting it's a faulty raid controller. If this is the case, is it possible to change the raid controller? I am about to order several Terastation Pro II units for my business, but am alarmed about how many instances of raid controller failures I've seen. I would just like to know if there is any recourse in the event the HDs are not the problem.
Thanks!