The ReadyNAS Duo comes in three flavors; 500GB, 750GB and 1000GB storage capacities. I'm testing the ReadyNAS Duo RND2150, which came with a pair of Seagate Barracuda 7200.10 SATA 3.0Gb/s 500-GB hard drives that Windows detected as having a capacity of 461 GB.
Netgear ships the Duo with a single SATA hard drive already installed in drive bay 1, thus it isn't a BYOD NAS. Users can add a second drive to their Duo (Netgear provided the second drive for our testing purposes), which will automatically be detected and configured as a redundant drive using Netgear's XRAID technology, or Expandable RAID technology. XRAID is similar to RAID 1, allowing for data redundancy over two disks, but providing no additional capacity.
For the second disk, you don't have to add the same make and model of the existing disk, but it is recommended. Netgear has a hard drive compatibility listing, as well as other supported hardware, located here. Upon adding the second drive, XRAID will automatically make a duplicate of all the data on drive 1 and run that second drive in redundancy mode.
The nice thing about XRAID is that it is automatic and requires no configuration on the end user's part. The bad thing about XRAID is there are no options to use other disk array types. A second drive can't be configured in a RAID 0, RAID 1 or JBOD array. For a home user who doesn't want to mess with these options, this makes the Duo more user friendly.
Update 4/15/2009: Can be configured for RAID 0
Due to user requests, NETGEAR added the ability to use the RAIDar utility to switch the Duo to Flex-RAID mode. In that mode, two drives can be configured for RAID 1 or RAID 0. The latter mode allows using a second drive for capacity expansion. But if you lose a drive all your data will be gone. JBOD or spanning mode is not supported.
Drive Fail Test
The Duo disk drives are hot-swappable, which I tested by removing the second drive while logged into the Admin screens. Upon removing the second disk drive, I received notification of a disk event that the disk had been removed from bay 2 (Figure 2).
Figure 2: Disk fail alert
Even with the second disk removed, the system continued to function. I was able to access data on the Duo's network shares with the disk removed. The status screens did reflect the fact that a disk was missing, however.
Removing and installing the disks was solid. Pushing a button and lifting the lever allows for sliding the hard disk tray smoothly out of the Duo's chassis as shown in Figure 3. Sliding the disk back into the chassis also was easy, giving the impression of quality construction.
Figure 3: Removing a drive
Upon sliding the second disk back into the chassis, the Duo quickly recognized and automatically started a resynchronization process. Although I hadn't changed any files on disk 1, it looks like any physical disk activity will trigger the synchronization process. It took nearly 3 hours for the synchronization process to complete, but the Duo was fully functional during this time.
I repeated this entire exercise, this time removing the first disk drive from the Duo. The performance was identical, with the Duo generating notification of a disk event, but remaining fully functional without the first disk and during the subsequent resynchronization. It took another 3 hours to resynchronize, but again the system remained functional.