iirc last time i did a clean install on my A1200 i booted upp os4 from the bootfloppy and installed OS4 and then i booted os4 from the harddrive and installed OS3.9,I think i did get a grim reaper at some point when installing 3.9 but i could continue.
I would create two or three partitions for AmigaOS installations. each one would be 1gb. then i would create a partition for apps and one big partition for mp3 and other downloads. i usualy also creates one small partition if i want to use diskcache in Ibrowse because it has happend in the past that ibrowse has screwed my partition with the cache even if it has to be 5 years or more since it happend last time
