This article addresses either failed HDDs/SSDs or failing HDDs/SSDs.  Failed HDDs/SSDs will show "Degraded".  Failing HDDs/SSDs will not degrade the pool but show in the Pool error counters/

Disk Failure and Rebuild

Pools normally show green and healthy 

After disk fails, Pool shows “DEGRADED” in red

Scroll down to see which disk group(s) have a failed drive

Click disk group TAB under Pool and open the DEGRADED Disk group, blink drive so you can physically locate it to replace it.  After replacing it shows (failed or removed)

If rebuild of new disk doesn’t happen automatically you can click the “Pencil” to the right and Replace

If you have multiple drive sizes ensure you are replacing with the same size disk and hit APPLY

Resilver (ZFS rebuild) will re-establish parity as defined by the disk group.  Ensure that the Pool is back to healthy after the resilvering: 

The event log will show these typical messages for disk failures