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 FeaturesDual-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
Size: 4096 GB
Hard Drives: 2
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.