I've connected my hard drive from PC to other Windows PC and installed a game from Xbox...

Hi and thanks for reaching Microsoft Xbox gamer's community. My name is Josef.
I'm an Independent Advisor and a Microsoft Xbox user like you.

Hope you're doing great today Alex

In this case you need to reinstall the Forza horizon game on the PC1
Because each PC has an unique ID and even if you’re using the same account the Xbox app is not going to recognize installed games from another computers

No I installed Windows 10 from my PC, I dont have it on a flash drive. BTW I would prefer it if i dont format my hard drives and reinstall windows 10

I reset my pc. I can download one other game. I reformatted the external drive and reset my pc but chose to reinstall Windows from my hard drive. I don't know if that formats the hard drive but I did chose the option of deleting all my files and keeping nothing. Still having the same problem.

I don't think that will work. Connected to a PC, you could only download PC games. They won't download as "XBOX" games.

For your scenario, you'd have to bring his XBOX to your house and connect it to the internet. Then connect the hard drive to the XBOX and download directly to the hard drive, from the XBOX interface.

Once you format a drive from the Xbox One, it becomes dedicated to only Xbox One games and apps. You cannot use it on your computer for other items nor can you install media (music or movies) on it. You can use another hard drive just for media, but only
use the standard PC format. You can use smaller drives than the required 256GB for a game installation drive and only need a USB 2 connection to use it as a media drive, which can also still be used on your PC (or Mac for that matter).