There are a few posts regarding this error printed by Data Lifeguard for DOS with no apparent resolution:
http://community.wd.com/t5/Other-Software-Accessories/Critical-Resource-Error/td-p/137020
http://community.wd.com/t5/Desktop-Mobile-Drives/DATA-LIFEGUARD-DIAGNOSTIC-cannot-run/td-p/388279
For my setup, I discovered that it was the presence of a PCI-X SATA adapter card (apparently 64 bit) that triggered this error.
Temporarily removing the card from the slot allows the WD DOS software to run.