Hrm, do you know why the users get a blank page? Is it a blank page or a "document contains no data" error? Is it because the page is trying to use new data and it's not there yet? Is the I/O from the copy statement just causing the page to slow down? Does the page not finish loading for some reason, leaving some </table> tags missing?

Maybe whatever you're doing is locking some tables so the pages can't access it and the queries hang while waiting for the process to be done. Are your pages pl/sql or some other? (perl, jsp, php, etc).

What do you mean by truncate and copy? :)