Never let it be said that I'm not open to new ideas. After my recent review of Ecrix's VXA-1 tape drive, a number of people asked why you couldn't just use hard disks for backup.
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I quickly responded with all the reasons that hard disks are a fairly poor option as a sole backup solution. To wit:
Cost: Hard disks are much more expensive than tapes.
Redundancy: A single backup isn't sufficient for a good backup strategy.
Archiving: It's easy to make an identical copy of a hard disk, but doing so loses the benefits of archived data.
Single Use: It's tempting to use a backup hard disk for storing original data occasionally, putting backed up data at risk.
Convenience: It's much harder to connect and disconnect hard disks than to insert and remove tapes.
Transportability: It's harder to take hard disks to another site for protection against burglars, fire, or even earthquakes (not that we ever have those in Seattle).
But as the discussion progressed, I became convinced that hard disks can be used in a coherent backup strategy, thanks to the rise of cheap, large, FireWire hard disks. The dealmac Web site recently found an 80 GB FireWire hard disk at MadLogix for $335. At that price you could buy three hard disks for about $1,000, which is a good bit less than the roughly $1,500 you'd pay for a VXA-1 tape drive, its bundled 33 GB tape, plus 11 20 GB tapes (for a three backup set solution of comparable capacity). Even if I personally wouldn't be comfortable buying from the vendor with the absolute cheapest price, inexpensive FireWire hard disks are also available from TidBITS sponsors APS Tech and Small Dog Electronics, along with ElectricDeal.com, a new company run by some old friends with a long history in the storage business.
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An even more convenient approach might be to use removable FireWire hard disks in a bay. For instance, Granite Digital sells kit parts for trays into which you can install inexpensive IDE hard disks; you can then slot these trays into frames that fit into 5.25" half-height drive bays in an external enclosure. (Click the Hot-Swap Bays link in Granite Digital's Web catalog). On the downside, the price of such a solution would probably be slightly more expensive than buying three stand-alone hard disks, and if the power supply in your external enclosure failed, all the hard disks would be inaccessible.
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Answering the Criticisms -- Anyway, in this sample situation, for $500 less, you end up with much faster backup media with no need to swap among four tapes. And once you eliminate the price differential between hard disks and tape systems, many of the other criticisms of hard disk backup systems fall away. Working through the list above:
Redundancy: When hard disks are as cheap as they are now, you can afford to purchase several to support a multiple backup set strategy. That's necessary for a good backup strategy, since it's all too easy for calamity to befall a single backup.
Archiving: Although a distinct psychological barrier remains when thinking about hard disks as write-once media, you should treat them as such for archiving purposes. Alternatively, occasional CD-R backups could also meet your archiving needs. You could even combine the two by using the hard disks for daily incremental backups, then using Retrospect's transfer function to move the archive to a stack of CD-Rs (you'd need about 120 CD-Rs for this, which would cost about $60 and take quite some time to burn). It remains important to use a real backup program like Retrospect or Retrospect Express that backs up multiple versions of files, rather than a souped-up copy utility that duplicates your original hard disk. Identical copy backups don't protect against corruption creeping into files, such as the databases used by many email programs. (This really happens, as was discussed at length in TidBITS Talk recently.)
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Single Use: Nothing prevents you from using one of your backup hard disks for occasional storage of original data, but it's a terrible idea to put your backups at risk like that. I recommend labelling the cases of the backup hard disks clearly to remind you of their purpose.
Convenience: FireWire hard disks are simple to connect to and disconnect from your Mac, which eliminates one of the barriers to using older SCSI hard disks for backups in the past. It's important to minimize hassle in a backup strategy, since the more hassle there is, the less likely you are to back up regularly.
Transportability: Having multiple backup sets enables you to rotate one backup hard disk offsite at all times, something that wasn't financially feasible before. Hard disks are larger than tapes, but that's mostly a problem if you use a small safe-deposit box for offsite storage.
Hard disks still don't compete against tape solutions if you have to back up a great deal of data (hundreds of gigabytes) to multiple backup sets (the more sets, the more tape makes sense). You'll have to run the cost per gigabyte comparisons for your situation yourself, but the difference between a $335 hard disk and $180 worth of tapes will eventually eliminate the up-front cost of the tape drive.
There are also two limitations in the current version of Retrospect that could come into play. First, to back up to a hard disk, Retrospect requires you to use a Macintosh File backup set, which means your backup can't span multiple hard disks. Second and more problematic is a limitation with how Retrospect stores its catalog files for Macintosh File backup sets. Retrospect stores the catalog data (the table of contents of the backup) in the file's resource fork, and Mac OS 9 and the HFS+ disk format don't support resource forks over 16 MB. The catalog size is related to the number of files backed up (not the amount of data), and creates a limit of between 75,000 and 95,000 files. As a workaround, you could create another Macintosh File backup set on the same hard disk when the first one fills up, and continue doing so until the hard disk itself fills up, after which you must decide what to archive permanently and what to use again. Dantz will undoubtedly address these concerns in a future version of Retrospect.
Realistically, though, installations with massive backup needs already have serious backup strategies and hardware already in place. (And if they don't, they're fools.) The comparison between tapes and cheap FireWire hard disks as backup media works best in situations with small to moderate amounts of data to back up. If you fall into that category and aren't happy with your backup strategy currently, take a look at the option of multiple FireWire hard disks.