Simple question about backing up images

[sumguy]

i understand about filevault, but i don't use it. i wanted to know about the disk images i create manually with disk utility, for sensitive documents. with tiger, i never unmounted them before doing a superduper backup. i assumed that when i saved a file on one, the changes were immediately written to disk. was that dumb? if not, do i need to start doing that now in leopard, if i use the new sparse bundle format? i want to make sure superduper is never backing up corrupted copies of the disk images, but at the same time it's a pain in the butt to unmount/mount them all the time if it's not necessary.

with regards to thunderbird, it only appends new mail to the mailbox text file. it never removes deleted mail or rewrites the whole file, unless you do "compact folders". well i guess it's up to me to experiment with that one, i'll let you know!