what do you mean by when an ssd is been mirrored

Solution
"RAID (originally redundant array of inexpensive disks; now commonly redundant array of independent disks) is a data storage virtualization technology that combines multiple disk drive components into a logical unit for the purposes of data redundancy or performance improvement.

Data is distributed across the drives in one of several ways, referred to as RAID levels, depending on the specific level of redundancy and performance required. The different schemes or architectures are named by the word RAID followed by a number (e.g. RAID 0, RAID 1). Each scheme provides a different balance between the key goals: reliability and availability, performance and capacity"

http://en.wikipedia.org/wiki/RAID

Illumynization

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Nov 12, 2013
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"RAID (originally redundant array of inexpensive disks; now commonly redundant array of independent disks) is a data storage virtualization technology that combines multiple disk drive components into a logical unit for the purposes of data redundancy or performance improvement.

Data is distributed across the drives in one of several ways, referred to as RAID levels, depending on the specific level of redundancy and performance required. The different schemes or architectures are named by the word RAID followed by a number (e.g. RAID 0, RAID 1). Each scheme provides a different balance between the key goals: reliability and availability, performance and capacity"

http://en.wikipedia.org/wiki/RAID
 
Solution
To expand on that, RAID is used for performance purposes. RAID is NOT backup. You should still backup your files even if you are running RAID setup.

By storing data across multiple drives, it allows for multiple access points and improves access time. In general, outside of RAID 0, you will end up losing a full drive's worth of space in a RAID setup for the RAID array. Please know that all drives in RAID should be the same capacity. If not, it will run them all with the LOWEST storage capacity among the array.