SOLVED! Ok. It is the system change, not the atalkd.conf.
Now running on a Macintosh SE System 7.5.3.
The disk is not locked anymore and I can upload to it! Happy with the result!
Not sure why I get this problem in the combination Debian Sarge (Debian 3.1) and System 6.0.8.
In Debian Woody (3.0) and System 6.0.8 I do not have this locked disk syndrome.