Cannot install macOS Sierra onto external hard drive

[desertman]

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?

 

[CoastalOR]

You need to run the full macOS Sierra installer app ("Install macOS Sierra" app, about 4.97 GB). It is not distributed from Apple in a dmg format.

Format your external drive for Mac OS Extended (Journaled) using Disk Utility. You can get the full Sierra installer from the App Store, it will download to the Applications folder, then automatically launch, and then point the install to your external drive. The Sierra installer app will automatically delete after the install.

 

[desertman]

Sorry, I was wrong about the dmg file. I have a "Install macOS Sierra" installer file which I saved when I originally installed Sierra on my Mac last year. It is 4.78 GB big. I guess it is still the "original" 12.0 file.

The hard drive is (of course) formatted as "Mac OS Extended (Journaled)." I even reformatted it when the installation did not work - does not help.

Since I have already macOS 10.12 Sierra installed on my Mac mini I cannot download a newer installer from the App Store (strangely enough, it is also not listed in the Purchased section of App Store). But (also kind of strange) I can download it from App Store on my MacBook which is happening right now. Let's see whether I then can use this new installer (macOS Sierra 10.12.5) to get Sierra installed on an external hard drive.

 

[Fishrrman]

Use either DiskMaker X or Install Disk Creator to make a bootable copy of the installer on a USB flash drive of sufficient capacity (I suggest 16gb).

Then try again. In my own experience, things went better that way...

 

[chrfr]

The hard drive is (of course) formatted as "Mac OS Extended (Journaled)." I Since I have already macOS 10.12 Sierra installed on my Mac mini I cannot download a newer installer from the App Store

Yes you can. Delete your existing installer, empty the trash, and then you'll have no trouble downloading the 10.12.5 installer.

 

[desertman]

With new installer (macOS Sierra 10.12.5) the installation was no problem. It seems that these macOS installer files have some kind of expiration date.