If you simply copy your files to an external hard drive and then delete the originals, you're gambling that the external hard drive won't fail. Over time, you'll lose that gamble. By contrast, if you use disk mirroring (RAID1) or disk striping (RAID5) to store your data across multiple drives, a single disk failure isn't a disaster, but instead an advance warning to install a new drive.
Accordingly, I would recommend a dual-bay NAS device for RAID1 disk mirroring as a minimum specification for anyone considering this technology. This means that if you need 1 TB of storage, look for a 2-TB NAS solution supporting two separate 1-TB drives.
While it may seem extravagant to purchase twice the hard drive capacity than you expect to need, just remember that for every gigabyte of RAID-protected data moved to your NAS, that's a gigabyte you can delete from your PC's primary hard drive, freeing up space for easier defragmentation and better performance. Your laptop's hard drive should be a digital briefcase for the files you're working on now, not a digital safe deposit box containing everything you've ever done.
Starting with RAID1 on your NAS, you can safely delete files on your PC, and then more easily upgrade operating systems, try new applications, or download new content without worrying about disk space. It's a liberating experience.
Although RAID5 costs more to implement up-front and has performance disadvantages relative to RAID1, it provides higher levels of data protection and becomes quite cost-effective for larger implementations. Here are some sample calculations comparing the QNAP TS-210 two-bay (using RAID1) with the QNAP TS-410 four-bay NAS (using both RAID1 and RAID5), with 1-TB and 2-TB Western Digital Caviar Green hard drives:
|NAS||Hard Drives||RAID||TB Available||Street Price||Cost per TB|
|QNAP TS-210||2x1TB||Level 1||1 TB||$420||$420|
|QNAP TS-210||2x2TB||Level 1||2 TB||$540||$270|
|QNAP TS-410||4x1TB||Level 1||2 TB||$720||$240|
|QNAP TS-410||4x1TB||Level 5||3 TB||$720||$240|
|QNAP TS-410||4x2TB||Level 1||4 TB||$960||$240|
|QNAP TS-410||4x2TB||Level 5||6 TB||$960||$160|
Prices vary widely by manufacturer and disk size, but the basic math points to buying the largest available hard drives that fit in a given unit. While RAID5 is less expensive as storage needs increase (not including the higher power usage), it's most appropriate for read-only applications such as media servers. If you expect to use your hard drive capacity for write-intensive applications such as content generation, file downloading, or database access, RAID1 may be a better fit.
Many vendors, mainly traditional storage companies such as Buffalo, Iomega, and Seagate, provide NAS devices with pre-installed hard drives. Other vendors, such as QNAP, Synology, and Thecus, require you to purchase and install hard drives from a list of compatible devices. That aspect shouldn't put you off much, since NAS devices have been designed to make it easy to install or replace hard drives.