Only say it after a full write and sync. Yes this could mean many minutes staring at a blank screen, you might add some progress messages if you want, but nothing that might fool the user into taking out his disk before it is safe to do so!
-- To UNSUBSCRIBE, email to debian-bugs-rc-requ...@lists.debian.org with a subject of "unsubscribe". Trouble? Contact listmas...@lists.debian.org
