Dell XPS 720 Owner's Manual page 31

If a drive failure occurs, subsequent read and write operations are directed to the other surviving drives. A
replacement drive can then be rebuilt using the data from the surviving drives. Also, because data is
duplicated on the primary and additional drives, four 120-GB RAID level 1 drives collectively have a
maximum of 240-GB on which to store data.
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Setting Up and Using Your Computer