A mirrored pair keeps the volume readable when one of its two drives fails, and that is the whole of what the level offers. It presents the capacity of one drive and shares its box with everything else that can go wrong.
A mirror is the simplest redundancy arrangement there is, which makes it the easiest one to describe honestly.
The single failure it covers
Two drives hold the same content. If one stops working, the volume stays readable from the other, and the machine goes on serving while a replacement is fitted and the mirror is rebuilt.
It also covers a subtler case: a member that returns unreadable data for a particular sector. The controller can take that sector from the other member, and good implementations record it.
What it costs
Half the raw capacity. Two drives present the space of one, and that is the price of the arrangement rather than an inefficiency in it. Parity levels give up proportionally less as the member count rises, which is why they exist.
What it does not cover, and this is the longer list
Deletion. Removing a file removes it from both members in the same instant.
Hostile encryption. Written through, faithfully, to both.
The enclosure. A controller fault, a destructive power supply failure or firmware trouble sits above both drives.
Theft and fire. Both members are in the same box in the same room.
A mistake during maintenance. Pulling the wrong drive, or rebuilding onto the wrong member, is a real and common way to lose a mirror.
The rebuild is the risky moment
When a member is replaced, the surviving one is read from end to end to reconstruct the new one. That is the heaviest work either drive will do, and it happens at the precise moment there is no redundancy left. If the surviving member has a bad sector somewhere it has never been asked to read, this is when it is found.
This is not an argument against mirroring. It is an argument for having a second copy elsewhere before the rebuild starts, and for replacing a failed member promptly rather than running degraded for weeks.
Where a mirror is the right choice
Two-bay enclosures, where it is the only redundancy level available and the alternative is no redundancy at all.
Machines whose availability matters more than their capacity — a share several people work from, a machine that runs backups for a household overnight.
Simple recovery is a genuine advantage where the enclosure writes a readable filesystem to each member, because a surviving drive can then be read in another machine without that maker’s software.
Where it is the wrong one
Where the goal is capacity, because half the raw purchase is spent on the arrangement. Where four or more bays are available, since parity levels give away proportionally less. And anywhere it is being bought in place of a second copy of the files, because it is not one and cannot become one.
The manufacturer’s own documentation comes first, and this page is not a substitute for it. Every rate, capacity, class figure, endurance rating, port speed and power figure on this page is reported from the manufacturer's own documentation, with its unit written as published, and named as such — none of it is measured here. A stated capacity is reported as the maker counts it, and is never restated in another unit, because the two are not the same figure. And the second statement repeated rather than assumed: a RAID level is not a backup. It keeps an array readable through the failure of a drive, and it does nothing about a file deleted by mistake, an array encrypted by ransomware, a theft or a fire. A second copy somewhere else is a separate decision, and nothing on this site describes a machine as a place where files are safe.
The questions that come up before an order
Can I read the drives individually if the enclosure dies?
Sometimes, and it depends entirely on how the enclosure wrote them. Some write a plain filesystem to each member, which another machine can read; others write metadata of their own that only that maker’s software understands. The maker states which, and it is worth knowing before you need to know.
Is a mirror faster than a single drive?
For reading it can be, because either member can answer. For writing it is not, since both members have to receive the data. Speed is not the reason to choose the level, and any figure quoted for it belongs to a particular controller rather than to the level itself.
Last reviewed 10 September 2026