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 (under AppleCare warranty), 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.
(Or you might drop your computer or spill a drink on it. If this happens, you'll feel bad: but if you have no backup, you'll feel really bad.)
The second most likely problem is mistakenly deleting files. Delete cautiously.
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.
(Apple used to sell the Time Capsule, a version of the Airport Extreme wireless router that also included a big disk drive. You could back up one or more Macs to this device; they shared the space on the disk. Unfortunately this product is no longer supported. Instead you have to buy a "NAS" (Network Attached Storage) box. My Time Capsule is getting old and I guess I'll be researching these devices. I'll let you know what I decide.)
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. (Starting with 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.)
If you delete a file, Time Machine notes when it was deleted, but does not delete the file's previous backups, so when you "enter Time Machine" to retrieve a file, you can look back in history to a time when the file existed, and restore it.
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. Selectto 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 at the bottom of the screen to have them restored.
If your computer's hard drive fails and you want to get all your files back. Usually, when this happens, you have to take your computer to the Apple repair shop and get a new hard drive, or a new computer. (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.
On a portable Mac running Lion or later, Time Machine will create hourly " local snapshots" when your computer is disconnected from its backup drive. Local Snapshots are periodically reduced to one per day after 24 hours, then deleted after a week. 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.)
Time Machine (starting with Mountain Lion) can rotate its hourly backups to more than one external disk. (Thanks to the late James Pond, who shared much useful info on OS X backup.) James Pond also wrote a useful set of Time Machine troubleshooting articles.
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, or to the cloud, or all three.
Sometimes Time Machine will tell you its storage is corrupt and you have to start over: it may say "" Time machine may also start the whole backup over when a new Mac OS version changes the backup format. If your Time Machine drive has to be replaced, or a computer repair changes your main logic board's MAC address, you will also have to start over with backups. 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. But if your only copy of a file is in Time Machine, you could lose it.
Besides using Time Machine, backing up to an external hard drive is a good idea. External hard drives are inexpensive (compared to the cost of your computer): occasionally, copy every file you care about to one. Use a utility like 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 make a clone backup of 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.
If you are copying information to an external drive, either hard disk or Flash memory, it is a good idea to encrypt its contents. This information comes from an anonymous poster on www.macintouch.com on 16 Dec 2014.
You should still occasionally back up important data to a write-only removable disc. Here is how to burn CDs or DVDs, using the Finder, to back up your precious data to disc. (If your Mac doesn't have a CD-ROM drive, buy an Apple external drive.)
(You can set up a "burn folder" that burns the same stuff each time.)
You may assume that you don't need to back up your iTunes library contents, because you can re-download it from wherever you bought it in case of disaster. There are some gotchas though: some media providers may change their rules, or not permit re-downloading, or you may need account or password data in order to get the data.
iPhoto and Photos files are backed up to Time Machine only when the app is not open, and as mentioned above, Time Machine may discard all your backups with no advance warning. You can also store your "Photo Stream" in iCloud, as described below. But it is a good idea to also back up your Photos or iPhoto library to external disk or DVD. Apple provides no simple way to do this; here is what I do.
You now have one (huge) disc image file that contains your entire Photos 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.
You can also back up files to a USB stick or SD card. Some recent models of MacBook Pro have a builtin SD card slot; other Macs would use an external card adapter. You can copy files to the external device using the Finder, as described above. External flash memory devices like this can hold very large amounts of data in a small space. A 128GB SDXC card costs less than $60. You can use a Micro SD card with an adapter. Here is an article on using the SD slot. You can simply plug in the USB stick or insert the SD card; it will appear in the Finder and you can drag files and folders to it.
If you want a simpler way to back up your files, you can create a little script. I run a shell script in a Terminal window that uses the rsync command to copy files I want to back up onto an external flash device. The rsync command comes with the (free) Xcode command line tools from Apple. rsync is very fast and doesn't copy files that are already there.
You can make a little AppleScript file (one for each SD card) that backs up selected directories to your card, and just double-click an icon to do a backup. For example, to back up the directory "Documents" to a card named "SDXC_01", create an AppleScript that says
on run do shell script "rsync -avz --exclude .DS_Store $HOME/Documents /Volumes/SDXC_01/" display dialog "done" end run
To back up the contents of "Documents",
You should store a backup copy of your files outside the house. That way if your computer equipment is damaged, lost, or stolen, you can get your data back.
OS X includes the ability to sync information to the Apple iCloud service, starting with Lion. 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 Photos/iPhoto pictures, mail, calendar, contacts, Safari bookmarks, and documents 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.
Apple's page about How to Back Up iCloud Data suggests ways to keep copies of your iCloud data somewhere else besides iCloud.
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 (the first 5GB is free). 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, Arq, etc.
You can use Dropbox to back up a few crucial files over the Internet 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. Understand that Dropbox is less secure than using JungleDisk. You can get Dropbox apps for many different operating systems, including Linux, Windows, and iPhone. 2GB of storage is free. The app is free. 50GB is $99/year. I use Dropbox for file sharing, but not for backup.
I have not tried these services yet. Both give you 5GB free, and can buy more. They may make sense if your computer has good Internet connectivity most of the time.
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.
Some OS settings are not restored when you reload a Time Machine backup. For these, you should also keep specific backup files.
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 setup has saved me from crashes and hardware damage.