Since I bought my iMac back in March, I have been interested in a RAID 1 disk array for both my data drive and my boot drive. Why? If my iMac needs service, I have no choice but to send it with its internal drive for repair. By booting off an external disk drive, I avoid that problem.

One of my colleagues recently told me that Other World Computing was having a sale on an external RAID-1 disk array enclosure for $89! These are ordinarily much more expensive than what I have been willing to pay. Other World Computing is now selling this same device for $149, which is still reasonable.

But what is a RAID device? RAID stands for Redundant Array of Independent Disks. A RAID device uses two or more disk drives in order to achieve redundancy, greater storage capacity, or both. There are several ways to use disk drives in a RAID device. They are referred to as RAID levels. Some of the more popular RAID levels are: RAID 0, RAID 1, RAID 5, RAID 6, and RAID 10.

RAID 5, RAID 6, and RAID 10 are very expensive. They are used in large corporate environments where they need dozens of terabytes or petabytes of disk storage to be protected from drive failure. RAID has been around for many years. For anyone who is curious, a detailed explanation of RAID levels is at:

http://en.wikipedia.org/wiki/Redundant_array_of_independent_disks

A mirrored RAID 1 device combines two disk drives such that each time a write operation is done on one disk drive, the device does the same operation on the other disk drive. One disk drive is said to "mirror" the other. As a result, all my data is backed up on a near instantaneous basis.

You may be wondering why you would want to use a mirrored RAID 1 array on your Mac. There are two reasons. First off, if you need a high degree of data protection, keeping your documents and other files on a mirrored RAID 1 array is the cheapest way to get near instantaneous data protection. Second, if you need your Mac to be highly available, such that a boot drive failure doesn't crash it, then using a RAID 1 device as your Mac's boot drive puts you in a position where you would need to experience two simultaneous disk failures before your Mac would crash. The biggest down side to a mirrored RAID 1 array is that you can only use half of the array's disk capacity because the other half is the mirror.

PLEASE NOTE: A RAID is NOT a substitute for doing regular backups.

Just as everything I create on my RAID 1 device is copied to the other disk drive in the RAID device, every time I delete something, the RAID device deletes it from the other drive. As a result, if I accidently deleted an important document, I would need to restore it from my Time Machine backup because it would be gone from both of the RAID device's disk drives immediately. There is a very slight possibility that the RAID device could fail, which would could conceivably cause both RAIDed disks to also fail.

After my RAID 1 device arrived in the mail, few weeks passed before I found a pair of 1TB disk drives for my RAID device. This is only an enclosure, so it didn't come with disk drives installed in it. Other World Computing does sell the same RAID 1 device with a pair of 1.5TB or 1TB disk drives in it, but the price is not cost effective. A few weeks later, I purchased a pair of no-name eSATA 1TB disk drives from zipzoomfly.com for $79 each

I didn't open the box with the RAID-1 device in it until the disk drives arrived. Much to my surprise, the box included just the drive enclosure, power pack, and FireWire 800 and USB 2 cables. There were no directions of any kind in the box, nor was there any software. I didn't care about the software because it was stuff I am not interested in using, but I was surprised that the box lacked any directions in it.

So what to do with this device? First, I slide the two disk drives into the enclosure and plugged them in. Then I plugged the device into my iMac's FireWire 800 port. The two disk drives showed up on my desktop after I initialized them in Disk Utility. What to do now? I used Disk Utility's RAID tab to form the two disk drives into a RAID 1 configuration. The two disks then combined into a single icon on my iMac's desktop under the name I selected for the RAID device. Great!

Next, I used SuperDuper to make a bootable disk drive out of the RAID device by cloning my iMac's internal drive to it. I then should have tested booting from it, but I didn't, due entirely to laziness. I then used SuperDuper to copy my external data drive to the RAID array. This took at least two hours. Of course, I told SuperDuper not to delete the other data on the RAID disk.

After the data was copied, I disconnected my old data drive and I set it aside to store off-site for safe keeping. I then tried to boot my iMac off my new RAID device by restarting and holding down the option key. No go. An icon for the device showed up as a boot option, but I couldn't click on it; it was grayed out.

Then I realized I should have formatted each disk drive individually and make one partition on each of them with the GUID bit set. Reinitializing the disk drives wiped out all the data I just copied to the RAID device. I plugged back in my data drive and started over by formatting both RAID drives separately. I then partitioned each drive with one GUID partition and formed the drives back into a RAID 1 array using Disk Utility. Finally, I copied all my stuff to the RAID device again. I could have avoided wasting my time if I had tried booting from the RAID device BEFORE I spent two hours copying my data drive to it.

The RAID device still wouldn't boot off the FireWire 800 port. So I posted an inquiry about it on the iMac community forum via Apple's web site. I received some feedback that the iMac's FireWire 800 "should be bootable," but I didn't find anyone who was actually doing it. I found several people who said it "should work." The new iMacs do not have FireWire 400, so I couldn't go that route. This RAID array has two FireWire 400 ports, in addition to one USB 2 port and one FireWire 800 ports.

After several days of going back and forth with people on Apple's iMac forum, I tried booting the same device off of my iMac's USB 2 port at the suggestion of one of the people there. Much to my delight, my iMac booted fine off the RAID array via USB 2, so I made my RAID 1 array my default boot drive. I figured I would just use it via USB 2, which I did for a couple of weeks.

Finally, I contacted NewerTechnologies' tech support via their web site to ask about the problem booting via FireWire 800. The next day, I received a setup script from NewerTechnologies' tech support. That script fixed the problem. My iMac now boots from my RAID-1 array via its FireWire 800 port.

One of the concerns I had when I first received this RAID 1 device was that it would be noisy. I keep my RAID array on a shelf above my iMac. It does make some noise, but it isn't loud. This device has four status lights on the front. If you buy one, you need to keep it in a location where you can see those status lights, as they are important.

All in all, I am satisfied with my RAID 1 device, but it would be nice to have an alert window pop up if one of the disk drives fails. I did get in the habit of checking the lights on the front of the RAID 1 enclosure every day. As long as the hdr1 and hdr2 lights are green, it is fine. If either of those lights is red, it means one of its disk drives has failed.

If you use a RAID 1 device, it is best to use the exact same kind of disk drive in both slots (i.e., same manufacturer, same model, and same capacity). In fact, that's true of any of these low cost RAID enclosures. You can conceivably use drives from different manufacturers or different model disk drives from the same manufacturer, but that tends to break or slow down the RAID array's mirroring software. Either way, the two disk drives must be the same capacity. You can't put a 500GB and a 1TB disk drive in a RAID 1 enclosure and expect the RAID device to keep their contents in parity. At best, you would be wasting 500GB if you did that.

If either of the disk drives fails, it is best to replace both drives because it will be difficult, if not impossible, to find the exact same model disk drives months or years after you purchased the original two in the RAID array unless you purchased a third disk drive with the pair that are in it. Most people do not have the money to buy a spare disk drive and let it sit in a closet for a few years until it is needed.

To replace a failed disk drive, you would take the good disk drive out of the RAID enclosure and put it in a separate eSATA enclosure, then put two new disk drives in the RAID array enclosure, then copy the data from the old working disk drive to the RAID device. You can then put the old, working disk drive in storage since it does have all your files on it. If you are like me where you use your RAID device as your boot drive, you would also need to boot off a different disk drive, which for me would be my iMac's internal disk drive, which I do intend to keep current with OS updates.

I am knocking one Apple off my rating due to the lack of directions on how to set it up and the lack of any software to alert me to a disk drive failure. Unfortunately, as far as I know, none of the other RAID arrays for the Mac has software to alert you to a disk drive failure in the device, so implementing that feature may not be possible for such software to work on a Mac, although Linux would allow it.

In case you don't know, you don't need a RAID device in order to get the benefits that such devices offer. Apple's Disk Utility has built-in software RAID 1 feature. You can take any two external disk drives and set them up as a single RAID 1 device entirely within software. It is a good idea to use the exact same type of disk drive for both RAID drives. Also, a RAID device that's set up this way is not bootable and it will cause a modest performance hit for applications that are heavily disk intensive such as iMovie. There is third party software that can make a bootable RAID device out of two or more separate disk drives, but it's not cost-effective and it is still slower than a hardware RAID device.

The other down side with hardware and software RAID to be aware of is that you are using more power. Using disk drives means using more power. Of course, the increased power usage isn't that much, but for those of you who are energy conscious, I figured it would be best to let you know about it. I am a "tree hugger" so when I am not using my iMac, I simply shut it down and turn off the RAID array.