After a few more days with Mac OS 10.5.2, there’s one new feature with a very high Wow Factor: Time Machine.

This ultra-cool automatic backup system is the ultimate answer to every computer user’s worst nightmare—an unexpected deletion or file corruption, or worse, hard drive failure. This is how it works:

Anatomy of a backup.
For the initial backup, Time Machine copies the entire contents of the computer to your backup drive. It copies every file exactly (without compression), skipping caches and other files that aren’t required to restore your Mac to its original state. Following the initial backup, Time Machine makes only incremental backups — copying just the files that have changed since the previous backup. Time Machine creates links to any unchanged files, so when you travel back in time you see the entire contents of your Mac on a given day.

Timing is everything.
Every hour, every day, an incremental backup of your Mac is made automatically as long as your backup drive is attached to your Mac. Time Machine saves the hourly backups for the past 24 hours, daily backups for the past month, and weekly backups for everything older than a month. Only files created and then deleted before the next hourly backup will not be included in the long term. Put another way: You’re well covered.

It really is completely automatic, and it really works; none of that is empty hype. If you’ve ever had a hard drive crash with inadequate backups, or a catastrophic file corruption problem in which your backups were also corrupted (I’m looking at you, Microsoft Entourage), you know that Time Machine makes it worth doing the upgrade all by itself.