Installing windows 10 on a particular setup

Hello everyone,

I am thinking of installing Windows10 via Bootcamp on a partition on my iMac. I already tried this yesterday and after waiting a legendary amount of time (3hrs 30 minutes) for the disk to be partitioned, when I reached the point during the w10 install where it asks to choose the partition where you want to install it, Windows refused to install on the Bootcamp partition with the message: "We couldn't create a new partition or locate an existing one".

After googling for a bit I saw that this might/is happening because of external drives connected via USB.

Now, getting to the 'particular setup' part of my question: i have an iMac 21,5 which came by default with a 1TB / 5400rpm HDD. So , since having a 5400 rpm disk on a computer nowadays seems to me like self-mutilation (thanks Apple), I decided to buy an external SSD and to move everything there and keep the internal disk as ...storage. So, here is where my problem is when it comes to 'removing all USB external drives' : since I am booting on it and using it as main disk, I can't really do that.

Also after bootcamp failed to install windows, he also reset my booting order and it booted on the internal drive - luckily I didnt format it or anything so i guess it still had the required OS stuff on it.

So my questions would be:

1. Does anyone have any safe suggestions on how to install windows while booting from the external ssd and keeping it plugged in?

2. I was thinking to remove the SSD and boot on the old internal drive and try to install with Bootcamp from there but i am wondering: would this possibly cause problems afterwards when I want to boot again from my external SSD?

Thanks in advance !