hello...
ya - by default, os x treats mounted external drives as owned by whomever is logged in at the time...
you can override this default by checking "ignore ownership on this volume" in the ownership and permissions...see here for more info:
http://docs.info.apple.com/article.html?path=Mac/10.4/en/mh670.html
this will ensure that your permissions are preserved across logins...
sitruc