adcdefg wrote:
I was able to get the "Install macOS HighSierra" app. That was only 20MB.
That happens sometimes, especially to those with slow download speeds. That means you got all but one of the pieces, but the most important piece that contains the actual OS files probably timed out before it had finished. The way it works is that it downloads into a temporary area of your boot drive and when complete is combined with all the other necessary files in your Application folder.
It might be possible to download that missing piece and put it where it belongs, but it's a very complicated process. I think it has only happened to me once and I just tossed the "mini-installer" and was successful the second time. That's what everybody else I know who had this problem did, also.