I have a problem with my fresh Debian installation. I cannot write on any USB stick.
Does your USB flash drive seem to be write protected and read only after a new installation of Debian? Please check if you have the following two lines in /etc/fstab:
/dev/sdb1 /media/usb0 auto rw,user,noauto 0 0 /dev/sdb2 /media/usb1 auto rw,user,noauto 0 0
If yes, remove them and you should be able to write on USB sticks again with your normal user account.