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I need to make fresh installation on my iMac 2012 of macOS Sierra. I already upgraded to that one, but I want to make a fresh install. I found several articles, but I am not sure if they right, because I think they have an additional drive to make an installation on (they talk about non-bootable USB).

As far I do the following:

  1. USB flash drive formatted to HFS+ Journaled + GUID
  2. Install on it Sierra files using: sudo /Applications/Install\ macOS\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/Untitled --applicationpath /Applications/Install\ macOS\ Sierra.app --nointeraction
  3. Reboot + using Command + R key to show menu.

I need to delete my HDD from Disk Manager, but I am not sure that my USB is boot-able and I can start installation from USB drive. Also I am not sure that I will not delete some important part from my HDD!

Can someone advise me how to do that?

Remark: I have only this Apple computer at my disposal.

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Step 3 is "wrong" in this case, if you reboot holding cmd+R you will reboot into the recovery partition.

What you should do is to reboot with the USB plugged in while holding the alt/option key to get to the EFI boot manager.

Then you select the USB media and press enter to boot from it. You will be presented with the installation wizard, at this point go to the top menu and select disk utility to erase the internal drive (choose GUID partition and HFS+ Journaled as file system).

Now you can close disk utility and proceed with the installation.

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