A mirror disk contains the same data as the original one, and is used as a backup in case the original disk is unavailable. The same term "mirror" is used on the internet as in "mirror site".

The procedure is described here: http://www.helpwithpcs.com/courses/raid-levels.htm
"Using a technique called disk mirroring (see below) the same data is written to both disks at a time, so if one hard disk crashes then the same data is available from the remaining hard disk."


Also,
http://www.boran.com/security/sp/coldmirroring20010306.html

"For some servers, I prefer to put system (and certain data) files on a "normal" disk and mirror to a second disk once or twice a week ("cold mirroring"). If the boot disk dies, we simply boot from the mirror disk. This solution is easier to understand, to recover from in a disaster scenario, and system disks can be more easily added/removed/changed."