Devices that do not meet these system requirements will no longer be guaranteed to receive updates, including but not limited to security updates. The web site to see is: Customers who use assistive technologies can upgrade to Windows 10 at no cost. Your device might malfunction due to these compatibility or other issues. (Microsoft will not check to see if you do). You do have to agree that you use 'assistive technologies'. This offer expires at the end of the year. If you do not have working Windows 10 iso file and you have activated your Windows 8.1 installation, then you can upgrade to Windows 10 for free. Actually, you can install Windows 10 without using any USB drives.You should use the Boot Camp Assistant to download the Windows Support Software, but from there you can create the USB installer without the aid of the Boot Camp Assistant. Your Mac already contains all the tools needed to create a USB Windows installer from an Windows iso file.You can use a Microsoft 'Media Creation Tool' to download the Windows iso file. In other words, you should not use a Microsoft 'Media Creation Tool' to create the USB Windows installer. All Microsoft 'Media Creation Tool's are incompatible with Apples way of installing windows.

You make no mention of creating space for the Windows partitions. The Boot Camp Assistant creates the partitions necessary for the installation of Windows.I assume this is what you meant by USB drive. To avoid performance and reliability issues, we recommend that you upgrade your operating system to Windows 10 or download Windows 8.1 for free. You see three options: Mac, Windows and the USB drive. Microsoft 365 Apps is no longer supported on Windows 8.You make not mention copying the Windows Support Software to the USB. I understand you dowloadeded a windows utility to create the bootable USB.Some comments on what you are attempting. From there you can just click on the setup application and upgrade to Windows 10. It has been a while since I use windows 8.1, but I believe you can mount the Windows 10 iso.

Upgrade windows 8.1 to windows 10