http://hg.slitaz.org/tazusb/raw-file/tip/doc/tazusb.en.html

"writefs

The command 'writefs' will take the current memory resident filesystem and create a "rootfs.gz" file. If your flash drive is mounted as /home (as it should be), the new filesystem will be copied to the drive for you, otherwise it is left on the root of the drive"

script of my tazusb writefs gzip command
===============================================================================
The command writefs will write an
Keeping current sound card selection...[ OK ]
Do you wish to remove the screen resolution (No/yes/exit) ? n
Keeping current screen resolution...[ OK ]
Creating rootfs.gz with gzip compression... Moving rootfs.gz to media. Remember to unmount for delayed writes!
mv: cannot preserve ownership of '/home/boot/./rootfs.gz': Operation not permitted
===============================================================================
Root filesystem size: 36.6M

"the new filesystem will be copied to the drive for you, otherwise it is left on the root of the drive"; it is not left on the root of the drive