When trying to install macOS Sierra from a dmg file on my internal hard drive onto an external hard drive I get a popup asking for my password which says "Install macOS Sierra is trying to add a new helper tool," and when I then put in my password and click "Install" I get the error message "An error occurred while preparing the installation. Try running this application again." I tried to install the software now several times from two different Macs and it does not work in always the same way. When I searched for the error message I got several results saying that I need to set my date via Terminal to solve this - but also this does not help. How can I fix this?