I finally managed to install Windows 7 on my iMac. This was not easy since nothing worked as expected.
- You need a fairly new installable ISO, wich you can download from Microsoft (X17-59885.iso worked for me: http://msft.digitalrivercontent.net/win/X17-59885.iso. Newer version (not tested) here: http://msft.digitalrivercontent.net/win/X17-58997.iso)
- Do every step of the bootcamp assistant separately.
- IMPORTANT: Let bootcamp reboot your Mac for installation, or it won’t work.