DATAPUISTOKEMISTI

 I wanted to add some redundancy by mirroring disk. So I did it. Notice "-s" in zpool attach command so resilvering is done sequentially.

So you screwed up and created zpool using /dev/sdb instead of /dev/disk/by-id/ata-* as a device? Then you tried to fix it and ended up with cryptic wwn- as device name. Here's dirty solution. Make sure to run those commands right after each other. Otherwise it will get imported back with wrong device names automatically -- before you have time to do it with correct names.