recently I spent some time dissecting the file structure of Stellation Two "THE MILL" 6809 card OS/9 disks and I came up with a pretty clear picture of how the OS9 RBF file system has been implemented on the Apple II.
Since there is almost no software available for the "THE MILL" OS9 implementation, I decided to create a MS-DOS software tool that converts "THE MILL" card OS/9 disk image files (those DSK backups created with Apple II ADT) to OS9 RBF format (tipically the TRS-80 Color Computer OS9 format) and back.
By converting the "THE MILL" card OS/9 disk image files to OS9 RBF format it is possible to use a bunch of software tools (namely "toolshed"), created for the TRS-80 Color Computer, that allow to manipulate OS9 RBF disk content at will.
So, now, one can convert a "THE MILL" card OS/9 disk image to OS9 RBF , transfer files to and from the OS9 RBF image, convert OS9 RBF back to "THE MILL" card OS/9 disk image and create a real floppy with ADT or load the disk to an emulated Apple II with the "THE MILL" card "installed" ( see MAME emulator ).
All the tools can be run from an MS-DOS terminal session, so batch command files can be created to capture directory listings, clear folders, transfer files etc. back and forth on the image disks.
I already tested some simple TRS-80 OS-9 (level I) software modules (ECHO or DIR for example) on the Apple II and they work (OS-9 is a standard operating system) but I doubt that all of the TRS-80 software will work on the Apple II; anyway, at least BASIC09 programs can be transferred and some Apple II OS9 libraries rebuilt for this interesting and (and at that time) quite advanced multitasking operating system.
The "toolshed" tools can be downloaded from this page and user manual from this page, while my Os9_conv tool project (written in C and built with Eclipse Neon.2 and Mingw32) can be found at this page and the tool itself and a README file with instructions and examples can be downloaded from this page