USB booting :: Read / Write on usb stick with user dsl

Hello all
Thanks for all your help on those forums.

My problem is :

I boot on a pendrive 256 Mo and that works great
I save my backup.tar.gz (backup/restore) on it and that works great too.
But now, i'd like to read/write at startup on this pendrive with the user DSL, wich don't have permissions.
It works with root.
Each time I change permission on foler "sda4" to 777 (even after unmounting it), the permissions are changing when I mount it, and the user DSL cannot write or browse the usb stick.

The question is :
How can I read/write/browse at startup on my bootable usbstick with user DSL ?

Thanks for all.
LB

Problem resolved.
The usb stick is mounted as readable on /cdrom directory, and sda4..... dont know.
thx

original here.