Test the external drive with SeaTools for Windows
Use the Long/Extended test, and if the test finds nothing wrong with the drive, test the RAM modules with a bootable CD containing Memtest86
. Download the CD Image (ISO file) from here: http://www.memtest.org/
to create the CD from the ISO file: http://www.filehippo.com/download_imgburn/
Boot your PC from that CD and the RAM test will run automatically and continuously until you press the <ESC> key. Four passes is sufficient to reveal any RAM errors if there are any.
Test each RAM module on it's own so you know which ones (if any) are faulty.