I have a 4 disc RAIDZ config running on Freenas 9.1. Earlier I noticed that one of the drives had been taken offline. I ran smartctl, and all the discs passed SMART. A few days ago the machine had been powered off while a windows client was writing to it (I don't have a UPS yet). I thought that may have corrupted the data, so put the drive back online through command line. The drive remained online without incident.
So, I decided to run a scrub to check for errors. The scrub is able to get to ~50% of the pool size before the Computer takes the drive offline on its own. No CKSUM errors are reported. SmartCTL still shows all drives passing. Zpool Status reports that the drive was taken offline by the administrator - obviously I haven't taken the drive offline myself, so I assume that maybe FreeNAS is doing it for some reason.
The drive appears fine, with no CheckSum errors. Why would freenas be taking it offline? Should I check any other statistics on the disc to check if it is a true failure?