7.7.2 Hard Disk drives  A common and affordable approach to data storage on disk is to connect to a cluster of HDDs (hard disk drive) arranged in a RAID array (see section 6.3.14 Hard Disk Drives). RAID level 1 is little more than two drives mirrored; keeping two copies of the data on different physical hardware; if one disk fails it is available on the other drive. Higher level RAID arrays (2 to 5) implement increasingly complex systems of data redundancy and parity checking that ensures the data integrity is maintained. The higher level RAID arrays achieve the same level of security as level 1, or mirroring, but with significantly less storage space. RAID 5, for example, may have a 25% storage loss (or less depending on implementation), when compared to 50% for RAID 1. Sophisticated arrays are widely available.