This OWC external enclosure is a common sight on the desks of Mac users with big storage needs. It’s a pretty standard 4-bay box, styled somewhat like a cousin of a PowerMac G5 or 1st generation Mac Pro. Inside are the usual options of RAID 0 to RAID 5 with a few additions like JBOD & RAID 10 thrown in for good measure. There are a few variations of this device but the back panels commonly have USB, Firewire, and eSATA ports for direct connection to a PC or Mac. There is no ethernet port on these drives which makes the Qx2 a DAS (Direct Attached Storage) rather than NAS (Network Attached Storage).
Aside from massive name, the OWC Mercury Elite Pro Qx2 also comes with a potentially huge amount of storage. Currently up to 32TB on the OWC store, but also available diskless or BYOD (Bring your own disks). With so much storage space, these drives often become the one and only repository for vast lumps of important data. The benefits of RAID give a false sense of security that the data is safe from drive failures. Unfortunately, there are a number of reasons why the RAID array alone will not protect from certain failures. Most of these failures can be overcome by us in our workshop, but they are not one-button fixes. It is helpful to understand why a seemingly rock-solid platform can be even more risky than a simple external USB drive.
Under common settings, the Qx2 will use RAID 5 for the array. With four 2TB drives this gives you a 6TB volume on a Mac or ~5.5TB on a PC, and can cope with a single disk failure. There is a lot of debate about how good RAID 5 really is for such large drives. In our example this means that if a single disk fails, it will need to be replaced, and then the new disk rebuilt with 2TB of data calculated from the other disks. This will take many hours, even under optimal conditions, but if anything goes wrong before it completes the array could stop showing up all together. At this stage, the data is probably recoverable but don’t panic. One wrong move and the data could be gone for good.
If the data is crucial then get assistance from a RAID recovery service now and you should get back all or most of the data.
If any disks are removed or replaced at this point the array could get reinitialised and either make the recovery more complicated or wipe the data completely.
Aside from all the problems with a RAID setup, the volume could also fail in the same ways that a standard hard drive can. There could be deleted files, a reformatted or corrupt partition, or even the RAID controller failure. RAID cannot protect against those types of failure at all.
Our first step would be making read-only copies of each disk in the array. This protects against further disks failing, and also allows us to work from copies without risking the original disks. In fact, once the disks are copied, we put the originals to one side and don’t touch them again until all the data is recovered and supplied back to the user.
Once we have our copies, they are loaded into our own hardware where we recreate the RAID in a virtual environment. Again, we don’t use the original hardware, as that may have been the root cause of the problem.
When the virtual RAID has been loaded and all the data extracted, the files are supplied back on whatever alternative storage is suitable, (not the original device!) Once the data has been delivered to the user, and backups made, the old unit can then be destroyed, or returned and reused.
Anyone using RAID on a regular basis should know that RAID is not a replacement for backups. If anything, the increased number of disks makes failure more likely. This needs to be addressed by either making backups to another device, or an online service (preferably both). You ideally want backups that keep historic versions of the files, so that inadvertently deleting a file or changing a file by mistake will not also replace the backup version.
If you are having problems with an OWC Mercury Elite Pro Qx2, give us a call or send a message via the form on this page. We give free advice and could help you avoid permanent data loss.
1. Macs now use 1000 bytes for 1KB but PCs use 1024 bytes.
2. Even RAID 6 does not solve the long time required to rebuild a disk, even though it allows for two disk failures.