A
Before acquiring a second-hand computer, you should have run Apple Diagnostics or the Apple Hardware Test, whichever is applicable.
The first thing to do after acquiring the computer is to erase the internal drive and install a clean copy of OS X. You—not the original owner—must do that. Changes made by Apple over the years have made this seemingly straightforward task very complex.
How you go about it depends on the model, and on whether you already own another Mac. If you're not sure of the model, enter the serial number on this page. Then find the model on this page to see what OS version was originally installed.
It's unsafe, and may be unlawful, to use a computer with software installed by a previous owner.
1. If you don't own another Mac
a. If the machine shipped with OS X 10.4 or 10.5, you need a boxed and shrink-wrapped retail Snow Leopard (OS X 10.6) installation disc from the Apple Store or a reputable reseller—not from eBay or anything of the kind. If the machine is very old and has less than 1 GB of memory, you'll need to add more in order to install 10.6. Preferably, install as much memory as it can take, according to the technical specifications.
b. If the machine shipped with OS X 10.6, you need the installation media that came with it: gray installation discs, or a USB flash drive for a MacBook Air. You should have received the media from the original owner, but if you didn't, order replacements from Apple. A retail disc, or the gray discs from another model, will not work.
To start up from an optical disc or a flash drive, insert it, then restart the computer and hold down the C key at the startup chime. Release the key when you see the gray Apple logo on the screen.
c. If the machine shipped with OS X 10.7 or later, you don't need media. It should start up in Internet Recovery mode when you hold down the key combination option-command-R at the startup chime. Release the keys when you see a spinning globe.
d. Some 2010-2011 models shipped with OS X 10.6 and received a firmware update after 10.7 was released, enabling them to use Internet Recovery. If you have one of those models, you can't reinstall 10.6 even from the original media, and Internet Recovery will not work either without the original owner's Apple ID. In that case, contact Apple Support, or take the machine to an Apple Store or another authorized service provider to have the OS installed.
2. If you do own another Mac
If you already own another Mac that was upgraded in the App Store to the version of OS X that you want to install, and if the new Mac is compatible with it, then you can install it. Use Recovery Disk Assistant to prepare a USB device, then start up the new Mac from it by holding down the C key at the startup chime. Alternatively, if you have a Time Machine backup of OS X 10.7.3 or later on an external hard drive (not a Time Capsule or other network device), you can start from that by holding down the option key and selecting it from the row of icons that appears. Note that if your other Mac was never upgraded in the App Store, you can't use this method.
3. Partition and install OS X
a. If you see a lock screen when trying to start up from installation media or in Recovery mode, then a firmware password was set by the previous owner, or the machine was remotely locked via iCloud. You'll either have to contact the owner or take the machine to an Apple Store or another service provider to be unlocked. You may be asked for proof of ownership.
b. Launch Disk Utility and select the icon of the internal drive—not any of the volume icons nested beneath it. In the Partition tab, select the default options: a GUID partition table with one data volume in Mac OS Extended (Journaled) format. This operation will permanently remove all existing data on the drive.
c. An unusual problem may arise if all the following conditions apply:
OS X 10.7 or later was installed by the previous owner
The startup volume was encrypted with FileVault
You're booted in Recovery mode (that is, not from a 10.6 installation disc)
In that case, you won't be able to unlock the volume or partition the drive without the FileVault password. Ask for guidance or see this discussion.
d. After partitioning, quit Disk Utility and run the OS X Installer. If you're installing a version of OS X acquired from the App Store, you will need the Apple ID and password that you used. When the installation is done, the system will automatically restart into the Setup Assistant, which will prompt you to transfer the data from another Mac, its backups, or from a Windows computer. If you have any data to transfer, this is usually the best time to do it.
e. Run Software Update and install all available system updates from Apple. To upgrade to a major version of OS X newer than 10.6, get it from the Mac App Store. Note that you can't keep an upgraded version that was installed by the original owner. He or she can't legally transfer it to you, and without the Apple ID you won't be able to update it in Software Update or reinstall, if that becomes necessary. The same goes for any App Store products that the previous owner installed—you have to repurchase them.
4. Other issues
a. If the original owner "accepted" the bundled iLife applications (iPhoto, iMovie, and Garage Band) in the App Store so that he or she could update them, then they're irrevocably linked to that Apple ID and you won't be able to download them without buying them. Reportedly, Mac App Store Customer Service has sometimes issued redemption codes for these apps to second owners who asked.
b. If the previous owner didn't deauthorize the computer in the iTunes Store under his Apple ID, you wont be able to authorize it immediately under your ID. In that case, you'll either have to wait up to 90 days or contact iTunes Support.
c. When trying to create a new iCloud account, you might get a failure message: "Account limit reached." Apple imposes a lifetime limit of three iCloud account setups per device. Erasing the device does not reset the limit. You can still use an iCloud account that was created on another device, but you won't be able to create a new one. Contact iCloud Support for more information. The setup limit doesn't apply to Apple ID accounts used for other services, such as the iTunes and Mac App Stores, or iMessage. You can create as many of those accounts as you like.
B
How to maintain a Mac
Make two or more backups of all your files
One backup is not enough to be safe. A copy of a backup doesn't count as another backup; all backups must be made directly from the original data.
Keep at least one backup off site at all times in case of disaster. Backing up to a cloud-data service is one way to accomplish this, but don't rely exclusively on such backups.
In fact, don’t rely exclusively on any single backup method, such as Time Machine.
If you get an indication that a backup has failed, don't ignore it.
Keep your software up to date
In the App Store or Software Update preference pane (depending on the OS version), you can configure automatic notifications of updates to OS X and other Mac App Store products. Some third-party applications from other sources have a similar feature, if you don’t mind letting them phone home. Otherwise you have to check yourself on a regular basis.
Keeping up to date is especially important for complex software that modifies the operating system, such as device drivers. Don't install such modifications unless they're absolutely necessary. Remove them when they are no longer needed. Before installing any Apple update, you must check that all system modifications that you use are compatible. Incompatibility with third-party software is by far the most common cause of difficulties with system updates.
Don't install crapware
...such as “themes,” "haxies," “add-ons,” “toolbars,” “enhancers," “optimizers,” “accelerators,” "boosters," “extenders,” “cleaners,” "doctors," "tune-ups," “defragmenters,” “firewalls,” "barriers," “guardians,” “defenders,” “protectors,” most “plugins,” commercial "virus scanners,” "disk tools," or "utilities." With very few exceptions, such stuff is useless or worse than useless. Above all, avoid any software that purports to change the look and feel of the user interface.
It's not much of an exaggeration to say that the whole "utility" software industry for the Mac is a fraud on consumers. The most extreme examples are the "CleanMyMac," "TuneUpMyMac," and “MacKeeper” scams, but there are many others.
As a rule, you should avoid software that changes the way other software works. Plugins for Photoshop and similar programs are an obvious exception to this rule. Safari extensions, and perhaps the equivalent for other web browsers, are a partial exception. Most are safe, and they're easy to get rid of if they don't work. Some may cause the browser to crash or otherwise malfunction. Some are malicious. Use with caution, and install only well-known extensions from relatively trustworthy sources, such as the Safari Extensions Gallery.
Only install software that is useful to you, not (as you imagine) to the computer. For example, a word processor is useful for writing. A video editor is useful for making movies. A game is useful for fun. But a "cache cleaner" isn't useful for anything. You didn't buy a computer so you could clean caches.
Never install any third-party software unless you know how to uninstall it. Otherwise you may create problems that are very hard to solve. Do not rely on "utilities" such as "AppCleaner" and the like that purport to remove software.
Don't install bad, conflicting, or unnecessary fonts
Whenever you install new fonts, use the validation feature of the built-in Font Book application to make sure the fonts aren't defective and don't conflict with each other or with others that you already have. See the built-in help and this support article for instructions.
Deactivate or remove fonts that you don't really need to speed up application launching.
Avoid malware
"Malware" is malicious software that circulates on the Internet. This kind of attack on OS X was once so rare that it was hardly a concern, but it's now increasingly common and dangerous.
There is some built-in protection against malware, but you can’t rely on it—the attackers are always at least one day ahead of the defense. You can’t rely on third-party "anti-virus" products for protection either. What you can rely on is common-sense awareness—not paranoia, which only makes you more vulnerable.
Never install software from an untrustworthy or unknown source. If in doubt, do some research. Any website that prompts you to install a “codec” or “plugin” that comes from the same site, or an unknown site, is untrustworthy. Software with a corporate brand, such as Adobe Flash Player, must come directly from the developer's website. No intermediary is acceptable, and don’t trust links unless you know how to parse them. Any file that is automatically downloaded from the web, without your having requested it, should go straight into the Trash. A web page that tells you that your computer has a “virus,” or that anything else is wrong with it, is a scam.
In OS X 10.7.5 or later, downloaded applications and Installer packages that have not been digitally signed by a developer registered with Apple are blocked from loading by default. The block can be overridden, but think carefully before you do so.
Because of recurring security issues in Java, it’s best to disable it in your web browsers, if it’s installed. Few websites have Java content nowadays, so you won’t be missing much. This action is mandatory if you’re running any version of OS X older than 10.6.8 with the latest Java update. Note: Java has nothing to do with JavaScript, despite the similar names. Don't install Java unless you're sure you need it. Most people don't.
Don't fill up your disk or SSD
A common mistake is adding more and more large files to your home folder until you start to get warnings that you're out of space, which may be followed in short order by a startup failure. This is more prone to happen on the newer Macs that come with an internal SSD instead of the traditional hard drive. The drive can be very nearly full before you become aware of the problem.
While it's not true that you should or must keep any particular percentage of space free, you should monitor your storage use and make sure you're not in immediate danger of using it up. According to Apple documentation, you need at least 9 GB of free space on the startup volume for normal operation.
If storage space is running low, use a tool such as OmniDiskSweeperto explore the volume and find out what's taking up the most space. Move seldom-used large files to secondary storage.
Relax, don’t do it
Besides the above, no routine maintenance is necessary or beneficial for the vast majority of users; specifically not “cleaning caches,” “zapping the PRAM,” "resetting the SMC," “rebuilding the directory,” "defragmenting the drive," “running periodic scripts,” “dumping logs,” "deleting temp files," “scanning for viruses,” "purging memory," "checking for bad blocks," "testing the hardware," or “repairing permissions.” Such measures are either completely pointless or are useful only for solving problems, not for prevention.
To use a Mac effectively, you have to free yourself from the Windows mindset that every computer needs regular downtime for maintenance such as "defragging" and "registry cleaning." Those concepts do not apply to the Mac platform.
A well-designed computing device is not something you should have to think about much. It should be an almost transparent medium through which you communicate, work, and play. If you want a machine that needs a lot of attention just to keep going, use a PC, or collect antique cars.
The very height of futility is running an expensive third-party application called “Disk Warrior” when nothing is wrong, or even when something is wrong and you have backups, which you must have. Disk Warrior is a data-salvage tool, not a maintenance tool, and you will never need it if your backups are adequate. Don’t waste money on it or anything like it.
