Using Boot Camp to install W10 on High Sierra 10.13.6

[tommy2k25]

I have been asked to install Windows 10 alongside Mac OS High Sierra 10.13.6 on an iMac 21.6" (2013 or later). I am using Boot Camp to do it, but even though I've formatted the USB drive correctly, and tried different formatting methods (exFAT, FAT32) (after downloading the correct version of Windows and pointing it to the right destination), it always tells me there is not enough space on the drive. I have used different capacities between 8GB and 128GB, but still the same message appears! I am running out of ideas. I have checked whether the computer is compatible and it is. Any ideas?

 

[MacsBug]

Boot Camp will not allow you to install Windows on a USB drive. It only allows for installing on the internal drive in your iMac.

The Boot Camp app will partition your drive, then setup the installer.

Will need a disk image of Windows 10 though before you start the process of setting up Boot Camp.

 

[tommy2k25]

Sorry, I should have made it clearer! I'm not installing Windows on the USB drive, I am installing it on the internal drive.
But it always says the USB drive (with the ISO files) on it is not big enough

 

[MacsBug]

Have you tried installing Windows 7 with Boot Camp and then inside of Windows upgrade Windows to Windows 10?

I am wondering if Boot Camp in High Sierra may not with Windows 10 since it was sealed several years after High Sierra was,