As stated in an early message, I've solved the backup problem.
export BACKUP=0
thanks to this instruction I now backup manually and
If your backup is slow, check its size, for example the bigHomeFiles tab in the control panel stats.
after that checking and the removing of big unneded files, now it takes about 5 seconds to complete the task. When I started this topic it took about 25.
My USB stick is already formatted as ext2.
thank you all,