One of the reasons many of us use a Mac is that most of the time, to coin a well-worn phrase, it just works. We don't have to spend hours dealing with driver updates or fixing problems caused by bloatware.
Occasionally, however, even on a Mac, things do go wrong. Fortunately, OS X comes bundled with several tools to help you diagnose and fix problems very easily. The two most useful are Disk Utility and Activity Monitor. Here are a few ways you can use those two utilities to identify why your Mac is not working as well as it should and then resolve the issues.
Read more: 5 steps (and 8 checks) to fix a frozen Mac
1. What to do when you Mac won't start up - how to fix your startup disk
A Mac that won't start up is one of the most frustrating and frightening problems most of us encounter, but it's usually easily fixed. First, you need to reboot your Mac from its Recovery Partition, if you're running OS X Lion or later. If you have an earlier version of OS X installed, boot from its install DVD.
If you're using the Recovery Partition, reboot while holding down Command and R. If you're booting from an install DVD, hold down C while it restarts. When your Mac has booted using the Recovery Partition, you'll see an Utilities Window. Select Disk Utility. If you've booted from a DVD, choose Disk Utility from the Install menu.
Click the First Aid tab and then click on your your startup volume (Macintosh HD, unless you've renamed it). Make sure you click the volume, and not the disk at the top of the list. Click Repair at the bottom of the window and Disk Utility will verify and repair the startup disk. When it's done. Restart normally.
Read next: How to reinstall OS X if your Mac fails
2. How to repair disk permissions
If your Mac starts up ok but some applications behave erratically, disk permissions, which control what each user can do with files and folders, may be to blame.
Thankfully there's a very easy fix, and again, it's Disk Utility to the rescue. Launch Disk Utility, either by firing up Launchpad and searching for it there, or by navigating to the Utilities folder (Command-Shift-U in the Finder) in Applications, and opening it from there.
Click on the First Aid tab. Click on your startup volume in the left hand pane. You now have two choices, you could click Verify Disk, which may save you some time if it turns out there's nothing wrong with the disk. But, as we're pretty sure there is something wrong, you can go straight to Repair Disk Permissions and click that. Disk Utility will start identifying and fixing problems, which could take a little while. You'll see a list of problems and fixes in the Disk Utility window as it goes through them.
Once it's done, you can quit Disk Utility and carry on using your Mac as normal.
3. Identify memory hogs
If your Mac is running slowly, there are a number of possible reasons and culprits. Your first port of call should be Activity Monitor, stashed in the Utilities folder in Applications. Launch it and you'll see five tabs along the top (four if you're running Mountain Lion or earlier), and a list of processes underneath. Click on the Memory tab. Look at the bottom of the window and you'll see a graph of memory usage. If it's green, everything is working as it should be. The tables on either side show you how your Mac's memory is being used.
OS X, in addition to using physical RAM (Physical Memory) itself, allocates some to apps (App. Memory), and also uses it to cache recently used files File Cache). If it starts to use close to the maximum physical RAM available, it compresses data that hasn't been used recently to save space (Compressed). If it needs more it moves the compressed data to disk (Swap Used), and if it needs more still, it uses disk space as memory for applications (Virtual Memory).
If the graph is amber or red, OS X is having trouble managing memory, perhaps because an application or process is hogging it. To find out which process is at fault, look at the memory column. Make sure it's sorted by highest to lowest usage (arrow pointing downwards).
Ignore processes which have 'root' listed as the user and focus on those running from your user account. If you see an application or process that's obviously hogging memory, quit it by clicking on it, then clicking the 'x' icon in the toolbar. Don't quit 'root' processes.
It's a good idea to restart your Mac once you're done.
4. Find out which applications are hogging the CPU
Click on the CPU tab and you'll see information similar to that in the Memory tab. The graph at the bottom shows user (in blue) and system (in red) CPU usage.
If you see an application which is using a significant chunk of CPU cycles, quit it and you should notice a performance improvement in your Mac.
If a root process appears to be hogging the CPU, don't just quit it, it's usually a symptom of another problem. Google the process name and find out what it does. One regular culprit is 'kernel_task.' It represents OS X's kernel and handles lots of low-level tasks. If it's using more than a few % of processor cycles, it could be that you've installed a system extension, or other software that accesses the system, which is causing a conflict. In some cases, users have reported that the MacBook Air, when the ambient temperature is very hot, runs very slowly and this shows up as 'kernel_task' hogging CPU cycles. The only sensible solution is to move it somewhere cooler.
5. Check the energy use of apps
The Energy tab was a new addition in OS X Mavericks. It's useful if your Mac is running on battery power and want to keep tabs on which apps and processes are using the most energy. While the battery menu in the Finder's menu bar will tell you which apps are using significant energy, Activity Monitor shows you how much energy all open apps are using and how energy usage as changed over time.
6. Check disk space and network
These two tabs deliver similar information to the CPU and Memory tabs, showing you how much disk space and network bandwidth open applications and running processes are using. Disk is useful if your Mac is very low on disk space – although you should always keep at least 10% free – as it will show you which apps are using lots of disk space while they run. Network will show you how much data each app is reading and writing over the network, and displays a graph of overall network traffic over time.