WinToUSB is a free software that allows you to 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. If you have a Windows installation source (a DVD disc, an image file) and a valid Windows product key, you can use WinToUSB to install Windows on your USB drive.
WinToUSB's key features include:Easy-to-use wizard interface that provides step-by-step instructions for installing Windows on a USB hard drive or USB flash drive.
Install Windows from an ISO image or CD/DVD drive.
Use any edition of Windows 8(.1) to create Windows To Go USB drive.
Support for MBR and GPT disk layouts.
Don't need install WAIK (Windows Automated Installation Kit) or WADK (Windows Assessment and Deployment Kit).
Freemium
Whats new: >> Improved algorithm for cloning Windows between disks with different sector sizes
Fixed bug: Failed to download ISO files because Microsoft changed the website
Improved NTFS formatting algorithm
Fixed some bugs to improve product quality
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