As reliable as Macs may be, some do break down - just like televisions, microwaves, and most other electronic devices. But your Mac has an advantage that most other products don't: it may be able to warn you before a problem escalates from minor annoyance to complete disaster.
It can also help you figure out which of the many hardware components that make up your Mac is causing a difficult-to-diagnose symptom. You can try these preventative steps at home:
1. Monitor Smart warnings
Smart is the self-congratulatory name for a drive-monitoring technology that, with some luck, will warn you of an impending hard-drive failure. The technology is built into the drive; certain software can access the data and report the results to you. The best of these utilities are the ones, such as Julian Mayer's free SmartReporter, that continually check a drive's status and send you an immediate warning if a problem is detected.
Unfortunately, your Mac can access Smart data only for ATA and SATA drives, so forget about using it with your external FireWire or USB drives. And even when it's working as intended, Smart isn't all that smart. A drive can still fail without warning - which is why you still need to back up your data.
2. Check your Mac's memory
The symptoms of a defective memory module can be subtle. Everything may work fine most of the time until - bang! - a seemingly inexplicable crash or data loss occurs. That's why it pays to periodically check the integrity of your Mac's memory -- especially if you've recently added new modules. Kelley Computing's free utility Rember is an excellent choice.
It's based on a command-line program called Memtest, by Tony Scaminaci. While Memtest doesn't have Rember's user-friendly interface, it can be run in the Mac's single-user startup mode, a bare-bones environment that allows for a more thorough testing of memory. In either case, expect to wait several hours for dependable results.
3. Check other hardware
The installation disc that came with your Mac includes a special program called Apple Hardware Test. On Intel-based Macs, you access it by holding down the D key at startup when the disc is in the drive. The software runs a set of diagnostic checks on several of your Mac's hardware components.
Apple Hardware Test can't fix hardware problems - you'll still need to replace or repair defective parts. But it can help you determine whether a given problem is due to a specified hardware component.
4. Read messages
If you're still stuck trying to diagnose a strange symptom or error message, launch OS X's Console utility. From here, you can read the log files that OS X maintains. These logs record messages detailing almost every significant event that happens on your Mac, especially those that signal a problem. The two most critical logs are console.log (which opens by default when Console is launched) and system.log (which you can open by selecting Open System Log from the File menu).
Although much of the content of these logs will make little sense to most users, check especially for recent messages that refer to a hardware component or to a hardware-related technology (such as FireWire, USB or Bluetooth). Such messages can mean that the cited component(s) is a contributing cause of the symptom. For external components, you can easily confirm this by disconnecting a device and seeing whether the symptom goes away.