Arch based Linux & Win 10

I recently hooked up a external drive to sons Asus LP. He is running win10 ,so it failed to see
it.So I wiped the external and reconnected to the LP. I got the windows version of Etcher and Salient OS. All seemed straight forward so far. So rebooting to Usb stick,I then followed the steps and installed the distro to the external.As a side note,since this was my sons pc I checked and rechecked that I had option done right as to drive,grub,ect…After install I checked bios were right and that the boot was set to the external,then allowed it to reboot.So far so good.Was working fine.Opened Gparted and it showed that windows was there and that linux was on the external.Beautiful,works great like I have this before.

Now this the weird part.When I told my son he was p*ssed but once I explained everything all was fine.That is till he tried to boot into windows.It had been overwritten by Linux.It was still on the external but at the same time,somehow it was on the internal drive too.I have been going over and over the installed steps and can’t think of where everything went so wrong.
I have been checking microsoft and Arch wiki,but no results have come back.I even searched the open web and got no matches or clues.So I have turned here to seek some enlightenment .
Has this ever happened before to anyone else? I should say his LP is Asus with 4 year old intel graphics,8 gb ram.Very fast and smooth running.

PS.After much apoligies and grovelling I did reinstall win 10 2004.