From Mac Guides
Not all RAM is created equal. Some brands and types will work flawlessly, while others will not. Some memory is so bad that it's obvious: the machine won't boot, or crashes almost immediately. Others can be more subtle, silently corrupting your data in ways that you may not notice until it's too late. It is recommended that you test any new memory (or the existing memory in a new computer) as soon as it's installed. As with all electronics you should also consider the hardware burn-in period where most problems will first crop up. If you get through the first 30 days or so without issues, chances are you'll be good for years to come.
Apple Hardware Test
While the Apple Hardware Test CD or DVD that comes with every new Mac includes a memory tester, it is widely regarded as inadequate. Many machines with real memory problems will erroneously pass the tests done by the Apple Hardware Test.
Memtest86 performs a very thorough set of tests designed to uncover even the most subtle memory flaws. It is multicore and multiprocessor aware and scanning speed is reduced thankfully. This application requires the Mac is booted from a CD or USB device using an .iso image downloaded from the developers website. Not booting up the entire operating system allows the memory test to access the full memory of the Mac instead of just the unused portion as the case with other tools.
- Depending on the speed of your Mac, the amount of memory installed, and the number of test iterations, Memtest86 may take several minutes to several hours.
- Once you're finished, you can restart to a normal bootup by pressing the 'escape' key or power button.
Memtest performs a very thorough set of tests designed to uncover even the most subtle memory flaws.
- Memtest is a Command Line application, but the supplied documentation will walk you through the process even if you're unfamiliar with the Mac OS X command line.
- It is best used in Mac OS X's single-user mode. Before testing, restart your Mac in single-user mode, holding Command+S buttons while restarting.
- Run the program for several iterations, to make sure your RAM passes the stress test.
- Depending on the speed of your Mac, the amount of memory installed, and the number of test iterations, Memtest may take several minutes to several hours.
- Once you're finished, you can restart to a normal bootup by typing reboot at the command prompt.
There are some free alternatives to Memtest:
- Rember is a graphical user interface wrapped around an older version of Memtest. It may be easier for inexperienced users, but comes with a couple of drawbacks:
- It uses an older version of Memtest which may not contain the very latest memory checking algorithms.
- It must run while Mac OS X is fully loaded and using more memory, which means that it cannot test as much of your memory as Memtest can in single-user mode.
- Memtest is released under the free GNU General Public License, so you might be able to find an older version that is free to download somewhere else.
- You can build memtester from source by going through a simple "make" step, as described on the website and in its README file. Once built, you still need to boot into single-user mode by holding down Command ("Apple") and "S" buttons during startup, "cd" into where you placed memtester, type "top" to find out the amount of available memory, run memtester and reboot afterwards by typing "shutdown -r now"
- Swap Test - If you suspect a memory issue, one of the easiest and most powerful tests is to simply remove half (for example) of your RAM and run the machine with half installed. If the system seems fine then swap one piece for one and run it again. This simple method will definitively zero in on a faulty module, even where many of the other tests don't.
If you completed all testing with no errors, congratulations...maybe. There is still the possibility of intermittent, thermal or other issues not fully addressed in these broad tests. If there were errors, check your warranty status and/or seller's return policy. The memory chips should be exchanged for ones that work.