Backup of Apple III System Demo Disk

sorry for a bit offtopic.
I hope there are some Apple III users here, who can help me. I have a working original Apple III System Demo disk and I want to do a copy / backup for safety.

This disk seems to be copy protected.
I´ve tried ADT on an IIe and made a diskcopy to a PC and back. Also I´ve tried to copy with SOS 1.2.
The copied disk boots and stops with :"SYSTEM FAILURE $06"

I´ve also tried the diskimage "Apple3SystemDemo.dsk" from apple3.org

Does anyone know how to solve this problem ? Or an other forum where I can ask this question.

Thanks a lot.

Andreas

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