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At a Glance
Product unRAID Server OS 4.2
Summary Lightweight Slackware NAS distro that runs from a USB flash drive and sports a unique style of RAID on any Linux supported hardware.
Pros • Runs on anything supported by the Linux kernel
• Painless installation
• Polished web-interface
Cons • Only runs SMB/CIFS
• Pricey if you want to run more than three drives

Lime Technology's unRAID Server OS offers a lightweight package for a quick NAS setup with an innovative software RAID driver. Unlike traditional RAID modes, unRAID doesn't do any striping or mirroring. So you don't get the performance benefits of striping or the redundancy of mirroring.

Instead, unRAID stores parity data on one disk in the array. This configuration allows a single drive failure to be rebuilt, and allows you to add another drive of any size or speed (it must be smaller than the parity drive) to the array at anytime.

unRAID is built on Slackware 11, one of the oldest and most mature Linux distros out there, running a fairly recent kernel. unRAID comes in three flavors:

  • unRAID Server Basic (Free) - supports up to three drives but lacks user level permissions.

  • unRAID Server Plus ($69) - supports up to 6 drives and user level permissions.

  • unRAID Server Pro ($119) - supports up to 12 drives and user level permissions.


unRAID, as it comes from the factory, supports only one network protocol, SMB. A far-cry from the half-dozen or so offered by other open source projects such as FreeNAS. What's interesting about unRAID is the innovative unRAID pseudo-RAID driver that is built off the standard Linux RAID5 driver.

The driver essentially retains the parity functions of RAID 5 without implementing striping. This allows you to use any size, speed or type of drives (as long as they're the same size or smaller than the parity drive) since files aren't split between drives. Each disk is accessed individually while the parity data is stored on a single drive in the array.

The unRAID driver is released under the GPL, and although the source can't be downloaded directly from Lime-Technology's site, it's in the unRAID server distribution and you can incorporate it into any other RAID arrays not built around their specialized server.

unRAID supports the usual gamut of user level permissions, allowing you to create accounts and access rights on a user level basis.


unRAID requires a USB flash drive 128Mb or larger which needs to be a bootable Linux system. In Windows, download and install syslinux, a tool for creating a bootable Linux file system on a USB flash drive. From a command prompt, run the following:

C:\syslinux.exe g:

Where "g:" is the drive letter corresponding to your USB drive.

Name the USB flash drive "UNRAID", and copy over the unRAID OS.

Set your computer to boot from USB (usually found in the BIOS' hard drive boot priority menu), insert the flash drive and reboot.

Note that the BIOS on older systems might not have the option to boot from USB.

The bootloader page allows you to boot either unRAID or the very handy Memtest86+ memory diagnostic tool (a must have for system admins and system builders).

unRAID Bootloader
Click to enlarge image

Figure 1: unRAID Bootloader

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