Taking proper care of data is an unglamorous business. Although the capacity of modern hard disks is impressive, little thought goes into contingency plans.

Here we explain different options for backing up your data, including tape, optical disc and network-based storage. We also look into methods of organising your data to be sure you're getting the best value from your backups.

Taking backups is a form of insurance. Like any other insurance, you decide the coverage based on the risks, and a balance must be struck between the value of your data and the costs of backing up.

Hard disk failures are often at the top of people's minds when considering backups but, in reality, hard disks fail relatively rarely, and human error or errant software are much more typical causes of lost data.

System administrators say a backup isn't a backup until you've restored from it. Going through the motions of backing up data is no good unless you also restore that data. Stories of incomplete, corrupted or just plain empty backups are common, and it's rare to find a user who doesn't have a first-hand backup horror story.

Different situations call for different strategies and techniques. There is no magic bullet for keeping your data safe, but good planning and preparation maximise the chances of recovering from a catastrophic failure. You also win most from a few basic steps, which are easy to execute; you just have to actually do them.

What to back up?
Data falls into many categories. Some data - photographs, letters and so on - is unique, where other data - MP3 and movie collections, for example - could conceivably be replaced. That difference is important when creating a backup strategy; unique data is typically most important to back up simply because it's intrinsically irreplaceable.

Aside from keeping everything neat, categorisation is usually a must for another reason: big hard disks are too large to conveniently back up. A fancy high-capacity tape unit is out of the reach of most users and, even with 9.4GB double-sided DVDs and high-speed DVD burners, spending hours backing up 200GB of data to removable disks is no fun.

Not only is it impractical to back up a large hard disk, it also rarely makes sense. Typically much of the data on a system is non-unique, such as the operating system itself and application programs.

And taking a single gargantuan backup isn't just a waste of space; after an equipment failure you may find yourself restoring onto different hardware on which your archived OS install won't work. In fact, OS backups do have a place; but it's a different place to your documents and digital photographs.

Start at the beginning
Proper backups start with proper data organisation. If you have the opportunity to partition your hard disk, consider backups when you do it. Do you need a 20GB system partition, or would 8GB be adequate - which would likely compress to fit on a single DVD for backup?

Either way, try to keep your OS and programs and your data separate. There will be some exceptions - some application preferences, for example - but keep the bulk of your data on a separate partition or under a specific directory, such as a 'data' directory.

There was a time when you only required a data directory. You could simply back up the data directory and be done with it, since spreadsheets and text documents invariably took up very little space. These days it's more complicated, as multimedia has become a routine duty for desktop machines.

Digital pictures, music and video collections are what drive the demand for large storage, and these media files add complexity to organising your data. Media files, being intrinsically large, should be logically separated from smaller data such as word processor documents and spreadsheets.

Even with sophisticated modern packages basic word processor files are still relatively small - the Word file of this article comes in at around 150KB, for example.

Once you've separated your media, subdivide it further into original versus replaceable items. Even if it might be tricky to replace your MP3 collection, it's probably not impossible - unlike your daughter's digital pictures.

Making decisions about which data is most important can be tricky, but it's worth thinking about it early; the less you have to back up, the more flexible the options. As part of this process, consider what data you need to keep online.

For a home PC, archiving older or infrequently accessed data might not make much sense, since there's an overhead in keeping track of the data once you've moved it from your system to offline storage.

But for office servers much data that needs to be kept doesn't need to be instantly available, and instead can be archived to free up space for new data, and keep the overall amount of data on your systems at a manageable level.

Backup technologies
Most storage media is useful for housing backups, but some is largely specific to backup and archiving. Tapes, for example, don't have a role as online storage for workstations and servers, but are invaluable for archival use, and backing up volumes of data where removable disks don't provide sufficient capacity.

Available data volumes, costs and speeds of various technologies all come into play when designing backup systems, and you may find a mixture of different options serves you best. In choosing storage technologies also consider where you expect the backups to be restored.

You should have no trouble finding a system that can read a CD-R, but if you choose an esoteric tape format be sure you know how and where you'll be able to read its tapes if the worst happens.

Modern PCs often come equipped with a CD burner, providing a handy mechanism for backing up a maximum 700MB of (uncompressed) data at a time. If your data is well separated you may find a single CD has plenty of room to store your most vital information. Even if you need to use two or three CD-Rs or CD-RWs, it can be a good compromise.

Modern drives are very fast, and blank CDs are cheap. But the shelf life of burned CDs isn't yet well documented, and some drives have trouble reading CDs recorded elsewhere. Still, for speed and economy with modest data volumes, CDs are a good option.

The next step up from CD is recordable DVD, which is slowly becoming popular on the desktop. After years of format wars, many manufacturers now offer drives that will read all four of the popular formats (-R, -RW, +R and +RW), so it's no longer critical to choose your DVD format so carefully. The exception is DVD-Ram, which is rare and is read only by a handful of multiformat models and DVD-Rom drives, typically from Panasonic.

For backup purposes recordable DVD is like CD, just with a larger data volume. The common DVD formats offer 4.7GB per side, with double-sided discs now available. Similar questions about data shelf-life exist for DVD as CD, and are worth a little more consideration because 4.7GB is lot more data to lose.

Besides stock formats, there are several other choices. Zip and other removable storage is still popular in some niches, but unless you have a very specific requirement for compatibility isn't a great choice for backup. It's expensive - a blank 750MB Zip disk is much more expensive than a 700MB CD-R - and it's less commonly readable.

Any storage format that requires a proprietary drive to read is that little bit less versatile, and at lower data volumes there's no benefit to offset that failing.

As the growth in hard disk sizes has outpaced removable storage additional hard drives have become popular as a backup option. Many PC motherboards come with basic hardware Raid support onboard, and modern operating systems provide software Raid support.

The most basic Raid standard that provides data redundancy is Raid 1, which is simple disk mirroring. That's great for protecting against hardware failure of a single disk drive, but not much else.

A better choice is to have a second disk of equal capacity and mirror changes to it according to a schedule. That way you're also protected against accidental deletions and - if you disconnect the backup disk - virus activity and the like in the meantime.

Hard disks make good near-line backup storage because they're also much faster than tapes or removable media. But additional hard disks are also a lot more expensive by volume than CDs, DVDs or tapes.

If you're considering a backup hard disk, look at external models based on Firewire or USB2 full-speed (avoid USB 1.1 or 'high-speed' disks, since they are hampered by slow interface performance). With an external drive, an offsite backup is as easy as taking it from home to the office (or vice versa) each day.