A mirror and RAIDZ are both ways TrueNAS keeps your files alive when a hard drive dies - and both are routinely mistaken for a backup. This page explains each in plain words, what a drive failure actually looks like, and the three common disasters that sail straight past any RAID level ever invented.
Drive failure is not a question of if, only of when - so the pool layout decides what that day feels like. Choose between a mirror and RAIDZ with open eyes, and then hold one sentence separately in your head for the whole page: RAID keeps the NAS running; a backup keeps the data existing. They are different jobs with different tools.
Example of a healthy two-drive mirror - the layout that forgives one dead drive.
A mirror writes everything to two drives at once. Either drive can die completely and the NAS keeps serving files from the survivor while you shop for a replacement. You pay with capacity - two 4 TB drives give 4 TB of space - and you gain simplicity: mirrors are easy to grow later by swapping in bigger drives one at a time. For a two-drive home or office NAS, this is the layout to choose.
With three or more drives, RAIDZ1 spreads data plus one drive's worth of recovery information across all of them. Any one drive can die without loss, and you keep more of the total space than a mirror offers. The honest costs: rebuilding after a failure works the surviving drives hard for hours on large drives, and a second failure during that rebuild loses the pool. Bigger arrays deserve RAIDZ2 (two drives of safety) for exactly that reason.
TrueNAS flags the pool as degraded, sends an alert if you set alerts up, and keeps working. You power down, swap the dead drive for a new one of at least the same size, tell TrueNAS to replace it, and the pool rebuilds itself - a process called resilvering, which takes hours on multi-terabyte drives. During the rebuild your files are available but the pool has no spare safety left, which is why you do not 'get round to it next month'.
Notice the pattern: RAID answers exactly one question - a dead drive. Every other question is answered by snapshots and backups.
Every step shown live on screen - installation, pools, shares and the settings that matter.
▶️ Watch Full VideoTwo drives: mirror, no debate. Three drives and space matters: RAIDZ1 is reasonable, knowing a rebuild is its stressful moment. Four or more large drives: seriously consider RAIDZ2, so a second failure during a rebuild is survivable.
In TrueNAS, a single drive can be extended into a mirror without wiping it - one of ZFS's friendlier tricks. Going the other way, or changing RAIDZ layouts, generally means rebuilding the pool. Plan the layout first anyway; it is the decision everything else stands on.
Hours, scaled by drive size and how full the pool is - a large, busy drive can take the better part of a day. The NAS stays usable throughout. Do not power it off mid-resilver, and do not start one on the eve of a holiday.