NAS devices come in many different shapes and sizes, as well as with different capacities and features. But for the most-reliable type of storage solution, the NAS devices of choice should be a multi-drive unit with RAID support. It is important to keep in mind, however, that while RAID adds extra layers of fault tolerance in the form redundant data storage--thus making your data's storage more reliable--it is still not a replacement for backing up your data. The best RAID solutions offer easy-to-use and robust tools for doing both.
One great example of such a device is the Synology Disk Station DS409+. The DS409+ is a four-bay NAS device that holds up to four, internal SATA drives that can be set up in a variety of different RAID configurations. The DS409+ also includes an external eSATA port and two USB 2.0 ports, which can be used to attach external hard drives to for additional storage or for dedicated backups of the server; you can also attach up to two printers to the USB ports and use the DS409+ as a print server as well.
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