Just build a new pc, with drives from my old pc in there. I had already installed games on...

Not the answer I hoping to hear.

This may become a real problem for PC gamers using game pass as typically when we upgrade our PC we usually use our storage devices for the new builds. I don't have any issues with other PC platforms like Steam or Origin when comes to this.

I can put the drive back in my old PC and boot it, but what do I do then to change the files and prevent this from happening when I switch it back to my new PC. It seems the files are secured.

You would have to redownload them to your new PC. The licences for the games are for the old PC, hence why they do not show up on your new one.

Thing is I was reading threads before I posted.

But my xbox does read dics for games I already had on new hard drive before I transfered from old drive

and stuff from old drive does not show on my new drive but shows old drive has been wiped clean.

example I had played call of duty3 on xbox360 slim before transfer and information was on it`s drive so

I put it in tray it read it loaded game and I can play it. I had not download whole game on drive just saves rewards

But will not load new games and looks like what I had tried to transfer is lost.

The error does not refer to the fact that you already have it installed on another PC but that there is already an installed version on this PC, try first to install from the Microsoft store on the C drive please

Again, why would it need to read if it is completely installed? PCs, and the old 360 did not do this. the disc had to be in the drive to verify you own the game but it wasn't actually reading to streaming material from the disc during the game (if the whole
thing was installed that is).