One of the best things about Time Machine is that it does seem to have made less-technical Mac users pay attention to backing up their computers. They can spend around $100 on a 1TB external drive, plug it it, and let Time Machine take over. From there on, no intervention is usually needed.

The next best thing about Time Machine is that it makes it possible to recover any of several different versions of a lost or damaged file. The way Time Machine consolidates backups over time (hourly to daily, daily to weekly), so there's no guarantee that you'll be able to recover a file as it was at a specific point in time. And the quicker you delete a file after creating it, the sooner Time Machine will drop it from the backup.

Another issue is that Time Machine is very inefficient when it comes to backing up very large files such as databases or virtual machine images. The slightest change means the file must be backed up in its entirety, and that can fill your backup drive very quickly.

For example, if you have a 4GB Entourage database that gets backed up every hour, that can occupy 80GB or more for today's and yesterday's copies, then another 4G per day for the previous month. That's a running total of 400G before we reach the older weekly backups.

A 4GB virtual machine image could take up the same amount of space (assuming you have an application running all day, every day), which means your backup disk is going to be full in about a month.

Another issue revolves around how quickly you need to be able to recover from a major problem. While you can restore from Time Machine as part of the Mac OS X reinstallation process (eg, after replacing a failed hard disk), that's still a time consuming process.

If you had cloned your hard disk to an external drive, you could immediately boot from the clone and carry on working until you have time to replace the internal drive and reclone it.

But cloning isn't really a backup strategy, as it only provides very limited protection against issues like inadvertently deleting a file. Once you've cloned the drive for the day, the previous clone has gone.

And in the event of a serious incident such as a fire, you probably won't have time to remove your clone or Time Machine drives from the premises. That's when a remote backup system comes into its own.

My current strategy uses Time Machine to back up most of my data. The main exceptions are my Entourage database (copied nightly to an external drive via Apple's Backup utility, keeping around two weeks of backups), and media files such as podcasts and recorded TV shows (which I don't back up at all).

My work in progress and certain other important files are also backed up to MobileMe once a day, and I do an incremental backup of my iPhoto library to CD or DVD (using Backup) every time I import a batch of photos. (I really must get in the habit of sending additional copies on optical media to a relative for safe keeping.)

Remote backup isn't a complete answer for Australian users, largely due to the relatively slow connection speeds (roll on the NBN) and the tendency of ISPs to meter uploads as well as downloads.

Why don't I clone my hard drive? If my main computer fails, there are others that I can fall back on until it's ready for use again.

What's your backup strategy?