Although the official Windows 10 Media Creation Tool also enables users prepare the bootable USB, it can’t create bootable USB by burning an existing ISO image file. In other words, the Media Creation Tool requires you download a fresh copy of Windows 10 ISO image from Microsoft servers and then select appropriate options to burn ISO file to USB drive.
There are plenty of tools to prepare bootable USB to install, most users prefer using the official Windows 7 USB/DVD Download Tool to make Windows bootable installation media.
With default settings the official Windows 7 USB/DVD Download Tool formats the USB to make it bootable and also displays a warning before doing so.
Make bootable USB without formatting the USB drive
If you’re new to Registry Editor it is recommended that you make a restore point.
Step 1: Open Registry Editor by typing Regedit in the start menu search box and press enter or press Windows logo + R.
Step 2: Go to the following key:
HKEY_CURRENT_USER\SOFTWARE\Microsoft\ISO Backup Tool
Step 3: Select the ISO backup Tool key, on the right-side, create a new DWORD (32-bit) Value by right-clicking on the empty space, clicking New, and then clicking DWORD (32-bit) Value and finally, name it as DisableFormat.
Step 4: Double-click on the newly created DisableFormat value and change its value data to 1 (one) to stop Windows 7 USB/DVD Download Tool from formatting your USB drive while preparing the bootable USB of Windows 10/8/7.
To restore original settings, simply delete the DisableFormat value.