I use Windows 7 and have no intention of changing my main OS to 10. I do, however, have a second hard drive that I was hoping to install Windows 10 on to. Alas, none of my flash drives are large enough to put the windows 10 ISO on. The largest one I have is ~4GB and the Windows 10 ISO is ~5GB.

Is there a way to install Windows 10 on a secondary hard drive using an external hard drive, or would that require reformatting the external hard drive? Alternatively, is there a way to install Windows 10 on a secondary hard drive from a Windows 7 OS without destroying the Windows 7 OS?
I have tried using google to search for a way to use a portable hard drive as a flash drive to install it on a secondary hard drive, but everything I get is about installing Windows 10 on a portable hard drive itself. I could get a new flash drive for rather cheap from work, but they are closed by now and I was hoping to be able to get it done today.

Sorry if this is the wrong location on the forum or not appropriate for the PSO2 forum to begin with.