Woohoo, it finally worked
My buddy needed to prepone our date, so there was no time for me to download 10.4.6. First thing I tried was 10.4.0 again, but as you can see in the first attached pic, there was no options button and therefore no way to change the partition type.
Then we connected the SSD via an USB - SATA adapter thingy to his windows laptop. He formatted the SSD with a MBR map, then I booted with OS9, but Drive Setup didn't work*. I did a reset, tried 10.4 - but nothing either (same deal as in my first post). Probably because Drive Setup kind of messed it up and 10.4 then had nothing to work with. So we did the exact same thing again and tried 10.4 instead, but again no luck. The SSD showed up on the desktop (first time ever), but it couldn't be initialized because Disk Utility could not unmount the drive
My friend tried to unmount it with Terminal and also typed a few other weird letter combinations, but he was as clueless as I
Next try was to connect the SSD to my MacBook (again USB - SATA thingy) running 10.8.5 - I then used Disk Utility to format the SSD with an APM, no problem. Then by accident booted the G4 with the installed 10.3.9 (instead of the Tiger CD) opened Disk Utility and initialized the SSD with OS9 drivers and 3 partitions of 37GB each. This finally worked, although OS9 showed up some errors with First Aid on those partitions (something like B-Tree header yadayadayada). I repaired them and voilá - everything seems to be perfectly fine now.
I'm off to put the OS9 on it and install MOTU drivers, Cubase and such in a couple of hours...
I've attached a pic with the SSD info after the formatting with 10.8.5.
So here's a quick rundown of what worked in the end:
1. used a USB-to-SATA-adapter to connect the SSD to a MacBook running 10.8.5
2. Initialize the SSD with Disk Utility
3. Re-Initialize the SSD with Disk Utility of 10.3.9 to write the OS9 drivers and partition it the way I wanted
4. Use First Aid to repair those partitions (this last step seems a bit weird to me, but hey......)
I'm really thankful to you guys for helping me out
. Hopefully this thread can help others as well. I'll order myself a USB-SATA adapter thingy now - those are just a few bucks and true works of art
*actually, Drive Setup showed a different thing. Instead of <not initialized> as before, it showed <not activated>. But it still froze after I clicked "initialize"...