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Affordable Drive Mirroring Keeps Your PC Running

If you can't afford any downtime, backing up your data files may not be enough. New controllers keep a second drive ready in case of trouble.

By Jon L. Jacobi, PCWorld   

You're not stupid: You know you have to back up your data. But if your hard drive goes down, you're still going down with it, at least until you can buy a new one and get all your software reloaded. Well, not any more! Hard drive mirroring--which transparently maintains a second, dynamic "clone" of your primary hard drive--is now available for your desktop PC.

Until recently, you would have needed to install expensive SCSI equipment to do what you can do with two new EIDE products: Arco's $229 DupliDisk and Promise Technologies' $99 DataDefender. Both are mirroring controllers that use cheap and readily available EIDE drives. Add one of these controllers, buy a second drive, and voilà--instant peace of mind. If either mirrored disk goes down, your system will stay fully functional until you can replace the defective unit.

Into the Looking Glass

Promise's DataDefender is a PCI card that adds another EIDE channel, while the pricier DupliDisk plugs right into one of your motherboard's EIDE ports to work its magic. We tested both products using a Pentium MMX-200 system and two 10.1GB 5400 rpm Western Digital AC310100 UDMA-33 hard drives. Unfortunately, setting up the controllers was not as painless as we'd hoped it would be. After Arco's technician helped us diagnose a bad cable we had no problems with the DupliDisk, but the DataDefender's automated BIOS was too smart for its own good.

Most users are unlikely to experience the problems we had, which were caused by existing information written to our test drives by a Promise FastTrak controller we had tested recently (see link at right). The FastTrak makes two drives appear to be one faster, super-large drive. Though we had repartitioned and reformatted the drives, the DataDefender found the FastTrak information and automatically used the drives as the FastTrak would have. After contacting Promise, we were given undocumented information on how to delete the FastTrak data and testing proceeded smoothly from there.

Once DataDefender recognizes that it has a new pair of drives under control, it asks you a series of questions, including whether there is any data on the disk that you want to save. If you answer yes, all the data is copied to the mirrored drive, a procedure that took about 45 minutes for our 10.1GB drive. We used our system for an hour, then swapped the mirror drive with the primary and found it to be the exact duplicate it was supposed to be. As with the Arco product, if one drive fails, you'll need to repeat the rebuilding process to mirror the replacement.

The more expensive DupliDisk's biggest advantage is that once you've installed it and run its setup, it requires no device drivers--making it independent of the operating system. The setup program is written for DOS and supplied on a floppy that unfortunately isn't bootable. We used with our own boot disk, then ran the setup program. Arco is in the process of licensing DR-DOS to remedy this inconvenience.

The setup program is straightforward, taking you step-by-step through the procedure. However, copying your primary drive is much slower than with the DataDefender. The sector-by-sector copy of our 10.1GB primary to the mirrored drive took a whopping 3 hours and 56 minutes. DupliDisk, like the DataDefender, performed without problems, creating an exact mirror of our primary drive.

The Price of Redundancy

Both products work as advertised, but at a performance penalty. In our informal tests, DataDefender was 8 percent slower, and the DupliDisk 12 percent slower, than a single drive running from the motherboard's integrated PCI EIDE adapter. It's best to use hard drives of identical brand and capacity. You can use unmatched drives, but all bets are off on the potential effect on performance--and you'll limit yourself to the capacity of the smaller drive.

The DupliDisk's operating system transparency is a big plus for IS departments or for anyone who has to deal with multiple OSes. But for Windows users, the DataDefender does just as good a job and costs $130 less. You might also consider Promise's $149 FastTrak, which also supports mirroring. For its modest price increase over the DataDefender, it provides a versatile BIOS setup program that allows you to choose between faster performance with a virtual drive of doubled size--or the ultrasecurity of a mirrored drive.

At press time, Promise Technology's Web site (www.promise.com) was not functioning. You can reach Promise at 800/888-0245. You can reach Arco at 800/458-1666.