did some more test including one unintended error and found something funny.
i have applied the
updater script to my personal OS9 folder from my main mac again, but this time
on the mini itself.
in my personal OS9 folder things like quicktime and ATI are not at their default path; i removed them out of their folder.
the updater script seems to ignore the missing path at /extensions/ATI/ and just didnt copy the minified ATI files.
so, after rebooting into the new system, as exspected, the monitor went black right after the extensions finished loading.
but now to what is questionable.
to fix the wrong ATI drivers i booted into the vanilla mini OS, added the minified drivers to the new system folder, and booted into the new system.
now the monitor works in the new system - but the 2D accelleration is gone.
okay, maybe another user error i thought, and bootet back to the vanilla mini OS.
after the desktop showed up, the mouse now blinks and updates its position only every 2 seconds. finder windows take ages to draw and are almost inaccessible. when moving the mouse over a menubar menu, inside the opened menu the mouse speed is back to normal, out again over the desktop, and it runs in 0.5 "FPS" mode again.
only after another reboot the vanilla OS is now back again to normal operation.
regarding the new OS i have no idea what is wrong there which prevents proper 2d accelleration...
...
then i was playing a bit with the startup disk issue on a dual boot mini.
if you have 2 volumes on the internal HD and both have a Mac OS 9 folder, you can not boot from the second one via controlpanel, you must always use the boot manager, just as with an OSX/OS9 dual boot install.
the weird thing is, when you only have one system folder on the (technically) second partition (and no system on the first partition), THIS is found...