RAID Level Comparison Table

Minimum # Drives 2 2 3 3 4
Data Protection No Protection Single-drive failure Single-drive failure Single-drive failure Single-drive failure
Read Performance High High High High High
Write Performance High Medium Medium Low Low
Read Performance (degraded) N/A Medium High Low Low
Write Performance (degraded) N/A High High Low Low
Capacity Utilization 100% 50% 50% 67% - 94% 50% - 88%
Typical Applications High End Workstations, data logging, real-time rendering, very transitory data Operating System, transaction databases Operating system, transaction databases Data warehousing, web serving, archiving Data warehousing, web serving, archiving

Free external hard drive or usb flash with
each completed recovery

Features RAID 6 RAID 10 RAID 50 RAID 60
Minimum # Drives 4 4 6 8
Data Protection Two-drive failure Up to one disk failure in each sub-array Up to one disk failure in each sub-array Up to two disk failure in each sub-array
Read Performance High High High High
Write Performance Low Medium Medium Medium
Read Performance (degraded) Low High Medium Medium
Write Performance (degraded) Low High Medium Low
Capacity Utilization 50% - 88% 50% 67% - 94% 50% - 88%
Typical Applications Data archive, backup to disk, high availability solutions, servers with large capacity requirements Fast databases, application servers Large databases, file servers, application servers Data archive, backup to disk, high availability solutions, servers with large capacity requirements

Types of RAID Arrays

Types of RAID Software-Based Hardware-Based External Hardware
Description Best used for large block applications such as data warehousing or video streaming. Also where servers have the available CPU cycles to manage the I/O intensive operations certain RAID levels require.

Included in the OS, such as Windows®, Netware, and Linux. All RAID functions are handled by the host CPU which can severely tax its ability to perform other computations.
Best used for small block applications such as transaction oriented databases and web servers.

Processor-intensive RAID operations are off-loaded from the host CPU to enhance performance.

Battery-back write back cache can dramatically increase performance without adding risk of data loss.
Connects to the server via a standard controller. RAID functions are performed on a microprocessor located on the external RAID controller independent of the host.
Advantages Low price

Only requires a standard controller
Data protection and performance benefits of RAID

More robust fault-tolerant features and increased performance versus software-based RAID
OS independent

Build high-capacity storage systems for highend servers
RAID Arrays with different architectures can be similar, but each type also has its own "favorite" failures and different techniques to handle.
With redundant RAID 5 configuration, one drive can fall offline and the distributed parity can be calculated, on-the-fly, and the user data will be presented as if nothing is wrong, this is known as 'critical state'. Understanding these mechanisms is crucial for effective RAID data recovery, especially during critical states where timely intervention can prevent data loss.
Running critical, the server's performance will be degraded but will continue to function. In most cases, this condition will be recognized and the suspect drive will be replaced and the RAID will rebuild by design. If running critical and a second drive falls offline, there is insufficient parity information to calculate and the RAID will collapse and all data on the array will be inaccessible.
The first way to prevent permanently losing your data due to an incorrect rebuild is to periodically check the status of your RAID. You should ensure that all drives are functioning and attend to any problems that you notice while the array still functions. If you have any additional backups of critical data, it's a good idea to check them regularly and especially before you attempt to rebuild a damaged RAID.
Minimum RAID rebuild times are functions of several variables, including HDD capacity, HDD data rate, data bus bandwidth, number of HDDs on the bus and the on-going I/O load on the array. A 2 TB hard drive might take 40 hours or more to restore.
RAID manufacturers differ vastly in their designs of the internal components and circuitry of their hardware and an in-depth knowledge of these designs are crucial for successful RAID data recovery. But because manufacturers do not disclose this information, RAID recovery techniques  require many years of development and reverse engineering in order to determine which ones are the most effective.