By now, almost everyone understands the wisdom of performing regular back-ups of your hard drive. With the Macintosh, OSX, Time Machine can mostly automate this process.
I always back up to an external hard drive. This way, if the worst should happen, I can quickly grab the external and run. This has never happened to me, but fires, floods, earthquakes, lightning strikes, and natural disasters do happen.
I have a very erratic schedule so i rarely back up at the same time each day. I also never turn my external hard drive on except for back ups or the occasional recovery of a file when I need the previous version. This protects me from the rare, but still possible, destructive malware attack. A powered-off drive is less susceptible to power brown-outs (potentially more damaging than surges) and the lightning strike.
For about three years now, I've been using Carbon Copy Cloner, (CCC) a shareware product from Bombich Software (www.bombich.com). This nifty program allows you to either schedule back-ups automatically, manually, clone the entire drive or copy selected folders. It works on all the three Macs I have from a G4 PowerBook running OSX 10.4.n to the latest version of 10.6.n. With Intel Macs, you can actually boot your system from the cloned USB drive. Power PC Macs do not support booting from USB drives but will from a Firewire drive.
The first time you clone a large drive, it may take quite a while depending upon your processor speed and how much data is involved. Subsequent back ups are much faster as CCC only copies changed files by examining the metadata for files. It also deletes files on the cloned drive no longer found on the main drive. So, if you have files you want to save that are no longer on your main drive, they should be copied to a different drive or a partition on the external. As I have a 1.5 TB external divided into three partitions, this isn't a problem.
All back-ups are performed in the background with very minimal effect on system performance. I was doing a back-up that I began after I started writing this review and it is finished already. But I have only added a few music videos since yesterday, so it probably took longer to decide what to copy than it did to do it.
CCC has been totally reliable, updates for it are also free and install with the usual Mac "No Muss, No Fuss" method.
While many people are completely satisfied with Time Machine, for those of us with erratic schedules and a paranoid* approach to protecting the external drive and our data, CCC does a perfect job.
* Yes, I know I'm paranoid, but I sometimes wonder if I am paranoid enough? ☺