At the very heart of unRAID Server OS lies a traditional network-attached storage system that is focused on providing two essential services:  serving and protecting your data.  If your requirements do not involve adding any 3rd party applications to your system, you will have the lightest hardware requirements for usage. 

• For businesses:  server-class x86 64-bit CPU (e.g. Intel Xeon); 2 CPU cores
• For home users:  x86 64-bit capable Intel or AMD x86 CPU; 1-2 CPU cores

• For business users:  4GB dual-channel ECC RAM
• For home users:  2GB dual-channel RAM

• For businesses:  server-class motherboard (SuperMicro); Xeon-class CPU support
• Ports.  number of SATA ports for expansion capability.
• CPU support.  Make sure the motherboard you purchase supports the CPU you’ve selected.
• On-board gigabit Ethernet.  This is important; you won’t be happy with only Fast Ethernet (100Mbit).
• On-board video.  You need video to configure your BIOS and provide a console.  You could also just install a cheap VGA controller.
• Operating system bootable from USB Flash.  These days, virtually all motherboards support this capability.  One rule of thumb: if the motherboard includes on-board GigEthernet, then it’s probably new enough to support USB boot.
• PCI/PCIe device slots.  These slots can be used to install additional storage controllers that will add more SATA ports to your unRAID Server (consideration for future expand-ability).
• An internally mounted female USB port.  This lets you prevent your boot device from being accidentally dislodged while connected other cables / devices.

• Ease of disk replacement.  Make it easy on yourself to replace drives when they fail and get a system that has tray-less drive bays built into the chassis of the system.
• Placement / cooling.  If your putting this in the basement, it probably doesn’t matter how many fans you have in there or how loud it is, but if you’re putting it somewhere in earshot, consider a system focused on low-power usage and passive cooling (heat sinks).
• Rack vs. tower.  If you have an AV rack, note the depth of any rack mount systems you look to purchase as AV racks are only 2/3 the size of a typical server rack (19″ vs. 30″).  If you don’t have a rack, you should probably buy a tower.
• Drive cages.  If you can, go “tray-less” and our personal review is that SuperMicro makes the best cages, followed by iStar, then Icy Dock.  Only typically applicable to tower-based chassis (quality rack-mountable enclosures typically have built in tray-less bays).

• Check the forums.  Users love to discuss their experiences with various brands of storage devices.  Read up before making your selection.
• Size matters.  The larger a drive, the longer it will take to clear it as well as replace it in the event of a failure.
• Network performance and SSDs for cache-devices.  SSDs can go much faster, but a typical 1 Gigabit network limits write performance to ~125MB/s anyway (within HDD capabilities).

• Since peripheral bus speed quickly becomes a bottleneck for parity sync/check, if parity sync/check speed is important to you, try to use PCI-E disk controllers.
• For a less expensive solution, you can use PCI disk controllers.
• Your parity sync/check speed will not be as fast, but all other normal operations (read/write of media data) will be just as fast.
• Since parity sync/check can be done during times when your server is not being used, parity sync/check speed is less important.

• The general rule of thumb here is to not go cheap and make sure to use a single +12V rail design.
• This is because split-rail designs may not direct full power to the peripheral connectors used by the hard drives.
• There is a good article on the unRAID Wiki that discusses this further.
• Many obscure, intermittent hard disk problems may appear as you add hard drives in a system with a cheap power supply.