WinToUSB is a simple tool that enables you to easily install and run Windows operating system on a USB hard drive or USB flash drive, using an ISO image or CD/DVD drive as the source of installation.
WinToUSB claims to support Windows 10 Technical Preview too, next to Windows 7, 8, or 8.1 installations, but during my testing I couldn’t make the program create a bootable USB flash drive from the ISO file that Microsoft provided me with for being part of the Windows Insider program. It just displayed a rather unclear error message that didn’t help at all.
Anyway, though in my case WinToUSB failed to prove its reliability, it might still work in your situation. Maybe it didn’t finish its job in my case because I was testing its beta version. What I’m trying to say is that this tool shouldn’t be completely disregarded, as it comes with some really cool additional features. For example, it also supports creation of bootable WinPE USB drives, and it supports both the MBR and GPT disk layouts. Furthermore, it’s really easy-to-use and intuitive. Last but not least, it’s a free tool that comes with no price.
To sum it all up, it costs you nothing to give WinToUSB a try, and I think you should do that if you’re interested in creating Windows installations on portable storage devices.