It works exactly like before.Yyou need a boot.img and an installation.

It seems that your installation is incomplete or wrong (broken flags, missing files, etc)

Booting 1.4.5 hd install using the new boot.img probably will fail. Booting new MorphOS with the old 1.4.5 boot.img will fail for sure.

So, if booting the 3.7 CD works, you have installed the new boot.img and booting without fails because
it will try to boot 1.4.5 install, which fails. Make sure the boot priority of the morphos 3.x partition is higher than the 1.4.5 partition.