Of all the bad things that could happen to your computer, a disk crash is the most likely. A friend had a lot of great Photoshop pictures on her hard drive, and then one day her machine would not boot: the hard disk was dead, and she had no backup. She was lucky and got her files back. The same thing happened to me in October 2008; my 10 month old computer made a funny noise and wouldn't boot. All my files were lost! I got a new drive, restored from backup, and didn't lose a thing. If you have any important data that's stored on just one device, you should feel nervous. Hard disks are built to last a few years but can crash at any time.

The second most likely problem is mistakenly deleting files.

• Do not delete system files and applications that you "think" you don't need: you might be wrong.
• Don't waste your time deleting a lot of little files.
• If you do delete files, back them up first.

See Frequently Asked Questions about Time Machine and Backup.

If you care about the files on your computer, do more than one of the following:

Backup to external hard drive

Buy a big cheap external drive (I like LaCie drives). Enable OSX  Time Machine. This will help if your hard drive crashes or if you delete something and then wish you had not.

What Time Machine Does

When you first start it, Time Machine copies all of your files to the backup volume. Then, every hour, Time Machine copies the files changed since the last scan to the external drive. (In OSX Lion, portable Macs can keep some snapshots on the local hard disk.) It keeps the old copies, up to one every hour for the current day, the most recent one for the previous 30 days, and as many weekly backups as it has room for on disk for previous months. (For example, my backup for this computer currently has about a year's changes.)

What You Can Do With Time Machine

Time machine will be useful in two situations:

• If you delete or mess up a file, and want to restore a version from the past. First, navigate Finder to a view of the folder where the file used to be. Select (time machine icon) ► Enter Time Machine to see a Finder-like view of past files. Use the slider on the right side of the screen to select a date, to see how your folders looked at any available past version, or do a Spotlight search to find older versions. Select one or more files and click Restore at the bottom of the screen to have them restored.
• If your computer's hard drive fails and you need to get all your files back. Usually, when this happens, you have to take your computer to the Apple Store and get a new hard drive. (I hope you bought AppleCare.) When you get your computer back, your Mac may ask if you wish to restore from Time Machine, or you can use Migration Assistant to restore your files. The most recent version of every file will be restored from the Time Machine backup.

Time Machine is not designed to provide "archival" storage that lasts forever. It takes and keeps snapshots on its own schedule, which you can't change, and it may not preserve the version you think is really important. Don't delete things from your hard drive because you think "Time Machine has them." Files you want to keep "forever" (like raw pictures) should be backed up to CDs or DVDs, or to a separate hard drive. Sometimes Time Machine will tell you its storage is corrupt and you have to start over: it may say "Time Machine completed a verification of your backups. To improve reliability, Time Machine must create a new backup for you." This isn't much of a problem, if you are mostly using Time Machine to protect against disk crashes: you have one unprotected period until the first backup completes, and you lose the ability to undelete. If your only copy of a file is in Time Machine, you could lose it.

On a portable Mac running Lion, Time Machine will create hourly "local snapshots" when your are disconnected from your backup drive. These snapshots are coalesced into daily and weekly snapshots just like regular Time Machine backups. If your disk starts to fill up, Time Machine will free up space by deleting old snapshots. These local snapshots are useful for getting a file back if you destroy it by accident, but they won't help if your hard disk crashes, because they are on the same disk. (You can turn local snapshots on or off with the tmutil command.)

James Pond has a useful set of Time Machine troubleshooting articles at http://pondini.org/OSX/Home.html.

Clone Your Hard Disk to an External Drive

External hard drives are inexpensive. Occasionally copy every file you care about to one. Use  SuperDuper or  Carbon Copy Cloner. Such a backup will be pretty safe if you then dismount the drive correctly, power it off, and put it away. (If you are really cautious, you could store the drive offsite.)

When you are installing a new major release of the operating system, or modifying your hardware, or taking your machine in to the Apple Store for service, you should clone your entire hard disk onto another external hard drive. Then, if anything goes wrong, you can restore the copy and be back where you started. If you ever suspect that your hard disk is failing, do this right away.

Backup to CD/DVD disc

You should still occasionally back up important data to a removable disc. Here is how to burn CDs or DVDs, using the Finder, to back up your precious data to disc.

1. Start by cleaning your CD drive.
2. Insert a blank CD-R, DVD-R, or dual-layer DVD-R into the drive. A dialog box will pop up. Choose open Finder and name the disc something like "tvv-20040131".
3. A Finder window will open. The new disc will show up in the bottom pane of the left sidebar.
4. Drag files and directories to the blank disc's name. You'll get a progress thermometer.
5. Gotcha: you can't just drag your home directory to the disc and hit Burn. Two reasons: one is that there are sometimes some files deep in your ~/Library/Preferences that are only readable by root, not by you, and the Mac DVD burning software will abort the whole burn rather than keep going. Second reason is that a few files in your ~/Library are created and deleted automatically by the system in the background, and if a burn sees any file vanish while it is running, it will fail. You can back up home directory files but don't try ~/Library; you will end up making a coaster.
6. A CD holds 650MB. A DVD holds about 4.29 GB, dual-layer about 8GB. (Dual-layer discs burn much more slowly.) If you click on the disk icon in the Finder window sidebar it'll tell you how much room is left at the bottom of the window.
7. When you've dragged enough, click the "Burn" icon. (Little Civil Defense symbol.)
8. Wait. Don't delete or move any files that are to be burned onto the disc until the burn finishes, or you will "make a coaster."
9. Eject the disc, write the date on it, and store it away.

(You can set up a "burn folder" that burns the same stuff each time.)

Back up iTunes

1. Run iTunes.
2. Create a playlist with everything in it.
3. Right-click on the playlist and select Burn Playlist To Disk. Select Data Disc. After you click Burn, it will ask if you want to make multiple discs if it won't all fit on one. Keep feeding blank discs till it's done.

Back up iPhoto

Apple provides no simple way to back up iPhoto to disk or DVD. Here is what I do.

• Attach an external hard disk drive.
• Using Disk Utility, create a new disk image on the drive. Use "sparse image" format with a large enough size to hold all your photos.
• Mount the disk image.
• Using the Finder, copy the Pictures folder from your home directory into the disk image.
• Dismount the disk image.

You now have one (huge) disc image file that contains your entire iPhoto library, including all metadata. You can split the .dmg file into multiple segments small enough to fit each on a CD or DVD using a Terminal command like hdiutil convert bigimage.dmg -format UDRO -segmentSize 650m -o splitimage.dmg and burn each to a separate disc. To restore, copy all the disc contents onto a hard drive, double click the first segment, and they will be reassembled. You will need enough free disk space for more than twice the size of your Pictures folder.

Offsite Backup

You should store a backup copy of your files outside the house. That way if your house blows away in a tornado or collapses in an earthquake, or your computer equipment is stolen by burglars, you can get your data back.

Backup to iCloud

If you are using OS X 10.7, "Lion," it includes the ability to sync information to the Apple iCloud service. You log into it with your AppleID. It gives you 5GB of free storage, and you can buy more. iCloud can store your iTunes music and iPhoto pictures, mail, calendar, contacts, Safari bookmarks, and document written by Apple iWork, and make them available on all your Apple devices. This is not a general tool for backing up all your Mac data files: it would not protect your Word and Quicken files, for example.

Backup to Amazon Cloud Storage Using JungleDisk

You can back up your Mac files to cheap cloud storage provided by Amazon Web Services. First, get a Simple Storage Service (S3) account from Amazon. Then, install  JungleDisk, a $20 Mac program that makes a backup copy of files you choose over the Internet to the S3 service. Once you set up JungleDisk, you can click an icon to run a backup anything that has changed: it has options to run automatically. I use this program to back up most of my hard drive a couple of times a month. I have configured JungleDisk to not use too much bandwidth, to avoid sanctions from the cable company. (It won't back up huge files, like Parallels disk images, which is OK with me.)

There are other cloud backup services that you could use instead... try Googling. I have not tried Carbonite, Mozy, etc.

Dropbox

You can use  Dropbox to back up files to the cloud manually. Just drag your file to your dropbox, and it will be copied over the Internet to an offsite server. This works for small numbers of files, and can be used even if you are traveling with your computer, away from your Time Machine but with good bandwidth.

Physical Copies

If your Internet connection is low-speed, it might take weeks to copy your files to a server in the cloud. In this case, you should burn a copy of your precious files to CD or DVD, or onto a portable hard disk, and put the copy somewhere safe. (For example, a safe deposit box, or a trusted friend's house.) You have to be systematic about doing this backup often enough that important files will be saved. Programs such as Data Backup can help you make these copies efficiently: it is shareware and is sometimes included with external drives.

Application Backup

Some OS settings are not restored when you reload a Time Machine backup. For these, you should also keep specific backup files.

• If you modify the Apache configuration, save a copy of your changed settings.
• If you install Perl modules, keep a list of what you installed.
• If you use Fink or MacPorts, keep a list of what you installed.
• Keep a list of the serial numbers of software that you have installed.
• If you use MySQL, make a periodic mysqldump backup of your database in a file.

Battery Backup

Speaking of backup... I use a desktop Uninterruptible Power Supply (UPS) to keep non-laptop machines, cable modem, and backup drive running if the power should flicker. This has saved me from crashes and hardware damage.