I just discovered what you're saying is correct and then read your post. I changed my code to show the ALIAS only. I changed it to the following, after I realized that the function is looking for the ALIAS name and not also the whole path.

IF !USED("CERTS")
   USE E:\PRODUCTION\DATA\CERTS EXCLUSIVE IN 0
ENDIF

>You're using it incorrectly. The USED() function expects an alias name under wich table has been open not the table name. The AUSED() function can be used to get the list of aliases under which particular table is open or you can chec its return value to see if the table is open already or not.