AppleJack has a hidden Expert Mode that can be accessed by typing “x” then pressing the return key.  Previous versions of AppleJack included an option to run memtest (to test for bad RAM) while in Expert Mode, however it only worked reliably with Tiger.  This hidden memtest feature has been updated, and according to my tests it now works great with Mac OS X 10.4-10.6.

To run memtest using AppleJack 1.6, you must first have preformed a custom install of AppleJack by clicking on the Customize button, and checking the option for “Memtest OS X”.  If you attempt to run memtest using AppleJack without previously doing this custom install, you’ll get the error “Could not find the required memtest program, are you sure it is installed?”.  If you have previously preformed an easy install of AppleJack 1.6 without memtest, you can simply run the installer again to gain this feature.

Instructions for running memtest using AppleJack 1.6:

  • Boot into Single User Mode (restart holding down command+s)
  • After the scrolling text has stopped and you see “root #”, type “applejack” and press the return key
  • After the AppleJack menu appears, immediately type “x” and press the return key
  • After the AppleJack Expert Mode menu appears, type “2” and press the return key
  • When asked if you want a log of the test results, type “y” and press the return key
  • Wait for memtest to run (about 2-6 hours based on the amount of RAM and speed of the computer)

Running memtest using AppleJack will run three complete passes on all the available RAM.  You should not run any other features in AppleJack prior to running memtest, it should always be done following a fresh reboot.

After memtest has finished testing the RAM, exit out of AppleJack by following the instructions on screen (“just hit return to quit”).  To view the results of the memtest scan after rebooting, see the /var/log/AppleJack.log file.