Dell XPS 720 (Mid 2007) - Manual (Page 31)

Setting Up and Using Your Computer
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.