Is your memory bad?
Congratulations. You just installed a new memory module in your Mac. Your computer’s RAM is now double what it was yesterday and it’s running faster than ever. The only problem is that it also crashes periodically and documents get mysteriously corrupted. You suspect a defective module is the culprit. But how can you be certain?
For starters, confirm that the memory you installed is (a) alive and kicking and (b) the right type of RAM for your Mac. Happily, checking both of these things is a snap.
Is It Alive? First, select About This Mac from the Apple menu and see how much memory it reports. If it’s not the amount you expect, your new RAM module is dead or you didn’t install it properly. To find out which is the case, go through the installation again, making certain that the module clicked into its slot properly. If it did and the memory still doesn’t register, you probably have a defective module.
If the amount of RAM seems correct, open System Profiler (select About This Mac from the Apple menu and then click on the More Info button). In the Contents pane, select Hardware: Memory. In the right pane, you should see a list of all your installed memory.
Check the Size, Type, and Speed columns. Make sure these specs match those of the RAM you intended to buy. To confirm that this type of RAM will work with your system, check your Mac’s documentation or use MacTracker.
Ah, but what if your Mac won’t start? In that case, pay attention to the startup sounds when you turn on the Mac. Unusual beeps instead of the normal startup tone likely mean defective memory. For details, borrow someone else’s Mac and check out Apple’s Knowledge Base article.
On Again, Off Again If all the specs check out, you may have the dreaded intermittent-memory problem—your memory module is weak but not dead. It works fine most of the time but has occasional hiccups. A hiccup at a particularly bad moment can result in anything from a corrupted document to a system crash.
What makes these problems so infuriating is that they’re so hard to diagnose. You have to wait for the next hiccup, which could be minutes or days away. Still, even with intermittent-memory problems, you aren’t helpless. Here are the things you can do:
Remove the memory you just installed (and reinstall any modules you took out to make way for the new memory). Wait a few days. If the symptoms disappear, a memory problem was the likely cause.
Get out the disc(s) that came with your Mac and look for Apple Hardware Test. With recent Macs, it’s on the Install and Restore DVD. With older Macs, it’s on a separate Hardware Test CD. To access Apple Hardware Test from the DVD, insert the DVD and restart your Mac while holding down the option key. A screen will appear, listing all bootable volumes. One of them should be named Apple Hardware Test. Select it and click on the right-arrow button to launch the utility. You will have a choice of a Quick Test or an Extended Test. Choose Extended Test.
One advantage of Hardware Test is that you can use it even if you can’t launch OS X. But if your intermittent problem is intermittent enough, Hardware Test may not test long enough to detect it.
For the final word, get KelleyComputing’s Rember. This utility can test your memory for any length of time. Enable its Infinite option, and it will test forever (or until you click on Stop). This allows you to run a test that lasts for days. After stopping, check the log output to see if it found any errors.
Emergency Test If you can’t even boot into OS X, try running Tony Scaminaci’s Memtest from the special single-user startup mode.
Nonstarting Macs The only problem with Rember is that you can’t use it if you can’t start up in OS X. One solution is Tony Scaminaci’s Memtest, a Unix command typically run from Terminal. (Rember is actually just a graphical front end for Memtest.) The advantage of Memtest is that you can run it from single-user mode, a special startup mode (accessed by holding down Command-S at startup) that may work even if your Mac can’t successfully start OS X.
Single-user mode offers another advantage: it allows you to test more of your memory. That’s because Memtest is unable to assess memory that’s currently in use by other software. After a normal startup (and a launch of Terminal), the memory accessed by the Finder and Terminal would be off-limits to Memtest.
To run Memtest in single-user mode (assuming it’s already installed), type
/Applications/memtest/memtest all 3 -l(check the user guide that comes with the software for more details). You can stop the test at any time by pressing control-C.
If any of the above paths indicates a defective memory module, contact the company where you purchased it and ask for a replacement.