Redundant disk systems reach small businesses. You can't prevent hard disk drives from failing, but you can protect your data by using redundant disks. Often used in servers but rarely desktops, Redundant Array of Independent Disks (RAID) systems write data to two or more disks at once, so if one disk fails your data remains available on the other(s), and your application keeps running.High cost has put RAID systems out of reach for most small offices. But Axiomtek recently released the Fastora line of low-end RAID products priced in the $1,000 range.\u00a0RAID offers eight levels of disk data duplication, but only three are typically used. RAID Level 0 treats two or more disks as one and improves performance by reading and writing data from multiple disks simultaneously. But RAID 0 won't protect you against a dying hard disk because files are written in only one place. What you want is RAID Level 1, and if you can afford it, RAID Level 5.First, a caveat: Don't use RAID in lieu of backup. RAID systems won't help you restore deleted files; once deleted, files disappear from all the disks.\u00a0\u00a0Axiomtek's Fastora NAS-T2\u00a0(network-attached storage) unit employs RAID Level 1, often called disk mirroring, which reads and writes the same information to two disks at once. If Disk 1 dies, Disk 2 carries on and your data remains available. This method doubles your storage cost per gigabyte, but is cheaper than RAID Level 5, which uses three or more disks. RAID 5 systems offer better performance, but prices usually start in the mid-four figure price range and go up quickly.Inside a small customized enclosure, the Fastora unit has room for two standard IDE hard disks (Integrated Drive Electronic disks used in most desktop systems) and includes management software.The size of a child's shoebox, the device includes two LAN connections, 10\/100Base-T and a 1000Base-T (Gigabit).\u00a0With dual 120G-byte drives, the unit costs $1,035. Or you can buy the NAS-T2 mini-tower without drives for about $700 and add your own.\u00a0Testing showed the management software provides better-than-average tools for most NAS systems to control users, set quotas on user disk space, send\u00a0 e-mails when status changes, and configure the system to support Windows, Apple and Unix\/Linux clients. After installation, the system was available and usable almost immediately from Windows 98\/2000\/XP and Xandros 2.0 (Linux) clients.Fastora includes its Data Replicator backup software for Windows clients, which leaves a utility in memory so files are copied whenever they change. You can synchronize backups between two NAS-T2s, but that's not as smart as backing up offsite. When you delete files, they're gone, although Fastora includes a way to keep multiple versions.For do-it-yourselfers, another option recommended by readers comes from\u00a0ReByte. The company's $150 product lets you turn any PC into a RAID Level 1 NAS system. Pop the product and another disk drive into an old computer to create a redundant-disk NAS.With ReByte you add a small board with an embedded Linux operating system accessed via Web browser to a Pentium II or better PC. Add one hard disk to make it a NAS, two to provide RAID Level 1, and three or four hard disks to provide RAID Level 5.I'll say it again: These systems don't replace backups, but they keep disks spinning when you absolutely have to keep your data available. And they do it for less money than ever before.