One of the 3TB drives started acted weird. I got emails from the smartd that the drive keeps failing the offline tests. I did a scrub – it found a couple errors. The drive went to WD for replacement, I got another one next day,
Two weeks later another drive, – a 2TB, – started misbehave. smartctl showed 300+ “Pending Sectors”. I took the drive offline and wiped it with zeroes using Disk Utility:
diskutil zeroDisk /dev/diska, the Pending Sectors count went down to 0 with no more errors reported. It looks like the drive fixed itself. I placed the drive back into the pool and reslivered it.