The hardware shop says that the faulty disk passes the Western Digital tests, and thus, that it is not faulty by their standards. Got two new ones, but decided to give the old disk a try first (see below).
As if the RAID problems were not enough, the wooden structure supporting the cluster gave way at it's base: take it apart, re-enforce the structure, put it back together again. At the end of the day, back to normal at least structurally. Then: remount the old disk, and start the rebuild.