Update:
tested an other usb-key. the same problem at all
an other test fails also: take an second usb-key, with only a 2GB ext2 partition. edit the syslinux.cfg "changes=/dev/sdc1" but also the same error occured:
setting up directory for changes
/dev/sdc1
testing the filesystem for posix compatibility
setup union directory (using aufs, with adba=none)
mount: mounting aufs on union failed
Fatal error occured - can't .......
a few tests later, i am testing the stick with the my BT2-UsbStick the BT2 changes partition does it!!
has nobody out there any solution, or can explain me the the errormessages coming up?
SOLVED THE PROBLEM: Manually create in ext2 partition the "changes" folder. now it works!



