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I am a PC guy trying to break into the Mac world. I want to buy a used Macbook off of Craigslist but I am concerned about the possibility that the previous owner may have intentionally or unintentionally installed malicious software like a key logger or trojan horse. I know Macs are generally safer but that doesn't mean there is no risk. If I am going to be doing things like online banking on this new computer, I'm not taking risks.

On a PC, I would reformat the HDD, reinstall Windows and reinstall each individual application using the original installation media. This is a tedious process, possibly taking several hours. Is this the best way to secure a Mac also? Since I am buying the computer used, I may not necessarily have the installation media. Is there a way to automatically reinstall all the applications (i.e. Time Machine)? Am I being overly cautious?

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I'd format it and put WINDOWS on it. Microsoft has been winning the security war for the past ten years! – Aaron Kempf Nov 5 '12 at 6:02
I'm definitely a Windows guy and plan to continue being one, but I'm starting a new business and I want to be able to be familiar enough with both platforms to service clients on either platform. In fact, I want a Macbook Pro so I can dual boot Windows and OSX. Hackintoshs seem like trouble. – Jeff Nov 6 '12 at 14:51
Not to mention they violate the Apple TOS and that's probably not a good way to start a business. – Jeff Nov 6 '12 at 14:52
up vote 1 down vote accepted

Reinstalling the OS without erasing will leave many sorts of third-party software (including malware) in place. What you want to do is start up from the the install DVD (10.6 or earlier) or recovery mode (10.7 or later), run Disk Utility (under the Utilities menu on the install DVD; in the main window of Recovery mode), erase the disk there (be sure to format it Mac OS Extended (journaled)), then quit Disk Utility and run the installer to install a completely clean OS.

The apps Apple supplies with the OS (such as Time Machine) will be reinstalled along with everything else. As for third-party apps, you'll either have to get the installer disks from the previous owner, or repurchase them. For apps from the Mac App Store, they're licensed to an Apple ID, which'll generally stay with the previous owner, so you'll have to repurchase them.

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That seems like it is certainly the safest and most tedious option. houbysoft said when I run the Internet recovery, I 'can be reasonably sure that there aren't any threats remaining.' Sounds like you are saying he is wrong. IS that correct? – Jeff Nov 4 '12 at 16:18
Yes; I just tested reinstalling, and all of my settings and third-party software remained (I did it from an InstallESD disk, not over the Internet, but the process is basically the same). I suspect some types of software and config would have been deleted/reset, but you certainly can't count on it as a way to remove suspect software. – Gordon Davisson Nov 5 '12 at 5:01
@GordonDavisson: again, it's implied that when you reinstall, you do a clean install. Obviously there isn't much point otherwise. The guide my answer links to says the exact same thing as your first paragraph :) – houbysoft Nov 6 '12 at 23:56

Just ask the owner for the original install DVD and reinstall the OS; that's pretty much the standard thing you do when you buy any used electronics.

Alternatively, if the laptop you're buying has OS X Lion or later, you won't even need the DVD, just follow this guide to reinstall the OS.

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…although if you can't trust what's on the boot volume, you can't trust what's on the recovery partition either. Even if you do an internet recovery, you can't be 100% sure it's not messing with whatever it downloads. – colons Nov 3 '12 at 0:38
@IainDawson: yes, yes, but then the DVD could be fake as well, etc... In general reinstalling from the volume should be reasonably secure. If you're that paranoid, it's probably simpler to just buy a new Mac. – houbysoft Nov 3 '12 at 0:40
@houbysoft thanks. that makes sense. is there any way to something similar with the applications or is an old fashion reinstall best for those too? – Jeff Nov 3 '12 at 0:44
@houbysoft also, if the os is reinstalled, can i be reasonably sure that any possible threats are removed or rendered ineffective? – Jeff Nov 3 '12 at 0:47
@Jeff: there is no need to reinstall applications; they are reinstalled when you reinstall the OS itself. As for the threats, yes, you can be reasonably sure that there aren't any threats remaining. For there to still be threats, the last owner would have to be extremely determined and knowledgeable. – houbysoft Nov 3 '12 at 0:49

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