Hi Stephan, you need a usb-stick with the installation for 10.11.0 and the update files for 10.11.1. The full installation you can only do, if you have saved the downloaded version from the app store before installation with DiskmakerX. The Update file 10.11.1 you can download from the internet at https://support.apple.com/kb/DL1845?locale=de_DE You can only downgrade an imac or Mac mini if a version < El Capitan has been installed before. If you have an imac or mac mini which is delivered with the installation El Capitan 10.11.2 it is not possible to boot a version < 10.11.2. If you try, you get the Disable sign when you choose the USB Stick or an external hdd. Did you have the installation file OS X El Capitan Installation (6.09 GB) ?