A RAID 1 creates an exact copy (or mirror) of a set of data on two or more disks. This is useful for set-ups where redundancy is more important than using all the disks' maximum storage capacity. The array can only be as big as the smallest member disk, however. An ideal RAID 1 set contains two disks, which increases reliability by a factor of two over a single disk, but it is possible to have many more than two copies. Since each member can be addressed independently if the other fails, reliability is a linear multiple of the number of members. To truly get the full redundancy benefits of RAID 1, independent disk controllers are recommended, one for each disk. Some refer to this practice as splitting or duplexing.

When reading both disks can be accessed independently. Like RAID 0 the average seek time is reduced by half when randomly reading but because each disk has the exact same data the requested sectors can always be split evenly between the disks and the seek time remains low. The transfer rate would also be doubled. For three disks the seek time would be a third and the transfer rate would be tripled. The only limit is how many disks can be connected to the controller and its maximum transfer speed. Most IDE RAID 1 cards use a broken implementation and only read from one disk so their read performance is that of a single disk. Some older RAID 1 implementations would read both disks simultaneously and compare the data to catch errors. The error detection and correction on modern disks makes this no longer necessary. When writing, the array acts like a single disk as all writes must be written to all disks.

RAID1 has many administrative advantages. For instance, in some 365*24 environments, it is possible to "Split the Mirror": declare one disk as inactive, do a backup of that disk, and then "rebuild" the mirror. This procedure is less critical in the presence of the "snapshot" feature of some filesystems, in which some space is reserved for changes, presenting a static point-in-time view of the filesystem. Alternatively, a set of disks can be kept in much the same way as traditional backup tapes are.

Also, one common practice is to create an extra mirror of a volume (also known as a Business Continuance Volume or BCV) which is meant to be split from the source RAID set and used independently. In some implementations, these extra mirrors can be split and then incrementally re-established, instead of requiring a complete RAID set rebuild.

A RAID 5 uses block-level striping with parity data distributed across all member disks. RAID 5 is one of the most popular RAID levels, and is frequently used in both hardware and software implementations. Virtually all storage arrays offer RAID 5. As with RAID 0, RAID 5 can be created with disks of differing sizes, but the storage space added to the array by each disk is limited to the size of the smallest disk—for example, if a 120 GB disk is used to build a RAID 5 together with two 100 GB disks, each disk will donate 100 GB to the array for a total of 200 GB of storage. 100 GB are used for parity information, and the excess 20 GB from the larger disk are ignored.

In our example below, a request for block "A1" would be serviced by disk 1. A simultaneous request for block B1 would have to wait, but a request for B2 could be serviced concurrently.

Every time a data "block" (sometimes called a "chunk") is written on a disk in an array, a parity block is generated within the same stripe. (A block or chunk is often composed of many consecutive sectors on a disk, sometimes as many as 256 sectors. A series of chunks [a chunk from each of the disks in an array] is collectively called a "stripe".) If another block, or some portion of a block is written on that same stripe, the parity block (or some portion of the parity block) is recalculated and rewritten. The disk used for the parity block is staggered from one stripe to the next, hence the term "distributed parity blocks". This means, of course, that the controller software becomes more complex.

Interestingly, the parity blocks are not read on data reads, since this would be unnecessary overhead and would diminish performance. The parity blocks are read, however, when a read of a data sector results in a cyclic redundancy check (CRC) error. In this case, the sector in the same relative position within each of the remaining data blocks in the stripe and within the parity block in the stripe are used to reconstruct the errant sector. The CRC error is thus hidden from the main computer. Likewise, should a disk fail in the array, the parity blocks from the surviving disks are combined mathematically with the data blocks from the surviving disks to reconstruct the data on the failed drive "on the fly".

This is sometimes called Interim Data Recovery Mode. The computer knows that a disk drive has failed, but this is only so that the operating system can notify the administrator that a drive needs replacement; applications running on the computer are unaware of the failure. Reading and writing to the drive array continues seamlessly, though with some performance degradation. The difference between RAID 4 and RAID 5 is that, in Interim data recovery mode, RAID 5 might be slightly faster than RAID 4, because, when the CRC and parity are in the disk that failed, the calculation does not have to be performed, while with RAID 4, if one of the data disks fails, the calculations have to be performed with each access.

In RAID 5, where there is only one parity block per stripe, the failure of a second drive results in total data loss.

The maximum number of drives is theoretically unlimited, but it is common practice to keep the maximum to 14 or fewer for RAID 5 implementations which have only one parity block per stripe. The reason for this restriction is that there is a greater likelihood of two drives in an array failing in rapid succession when there is greater number of drives. As the number of disks in a RAID 5 increases, the MTBF for the array as a whole can even become lower than that of a single disk. This happens when the likelihood of a second disk failing out of (N-1) dependent disks, within the time it takes to detect, replace and recreate a first failed disk, becomes larger than the likelihood of a single disk failing.

One should be aware that many disks together increase heat, which lowers the real-world MTBF of each disk. Additionally, a group of disks bought at the same time may reach the end of their bathtub curve together, noticeably lowering the effective MTBF of the disks during that time.

In implementations with greater than 14 drives, or in situations where extreme redundancy is needed, RAID 5 with dual parity (also known as RAID 6) is sometimes used, since it can survive the failure of two disks.