Figure out which disk is brokenIn my case this was easy, as the server is hosted, and the hosting company did a hardware check for me after random failures where the server suddenly died. They reported the following:
The requested check is finished now. We found problems with the drive with SN Z1F0WA1P so please confirm the complete loss of data on it and tell us when we may shutdown the server to change it.
Luckily I set up RAID-1 so the "complete loss of data" is not really a big problem.
I found the device id of the failed disk this way: