Be careful when buying RAID hard drive storage

As hard drive prices come down, very large hard drive storage becomes a reality. As of the date of this blog post, the largest single drive storage capacity drives are about 4 Terabytes (TB). Larger capacity drives are usually multiple-drive RAIDs. A Redundant Array of Independent Disks (RAID) is a combination of hard drives sold as one unit. So normally, a 4 TB RAID is really 2 - 2 TB hard drives working together. What is not normally explained in the ads is that there are 10 different RAID configurations and it takes careful reading of the specifications for any particular hard drive to figure out what is really being offered. For example, if there were an ad for this product, what would you think your were getting:

Western Digital My Book Live Duo 8 Terabyte Personal Cloud Storage NAS

You might think you were getting an 8 TB hard drive, but if you look closely at the technical specifications for this drive, you will quickly see the following:

Product Features

Dual-drive shared storage and wireless backup for PC and Mac computers
Secure remote access over the Internet from any computer
Dual-safe data protection with RAID 1 mirroring
WD 2go and WD Photos mobile apps for smartphones and tablets

Hard Drive
Size: 4096 GB
Hard Drives: 2
Manufacturer: Desktop

Ports and Connectivity
Network Connection: Ethernet 10 Mb-s

Cases and Expandability
Size (LWH): 6.86 inches, 11.6 inches, 9.06 inches
Weight: 5.78 pounds
Free Internal Bays: 2

Look again, what you are getting is 2 - 4 TB hard drives "with RAID 1 mirroring." So what is RAID 1 mirroring? Here is a link to a chart showing all of the RAID configurations. A RAID 1 configuration is (mirroring without parity or striping), data is written identically to two drives, thereby producing a "mirrored set." So the effective capacity of this 8 TB hard drive is really 4 TBs. The reason for the mirroring is to protect the data on the drive should one of them fail. But you can buy a 4 TB hard drive for far less than the price of one 8 TB RAID 1 configured drive. Don't be fooled into purchasing this purportedly 8 TB hard drive that will only hold 4 TBs. 

If you want a large drive RAID, you need to look for a RAID 0 configuration. This gives you a virtual drive that is the same as the capacity of all of the connected drives. Presently, RAID 0 hard drives sell for more than the combined price of the individual storage drives, but the size of the RAID 0 drives go up to as much as 20 TBs. Look at the chart referenced above and make sure you know what you are buying.