The feature that you are looking for is called "Windows To Go", fortunatelly, it is already available and you can find how to make it on a Flash drive here:
Creating the Windows 8 To Go USB Device:
- Windows To Go requires a specific partition setup in order to function. We will use diskpart to create the new partition setup. Launch an administrative level command prompt from the Start Screen.
- Make sure that your USB Drive is plugged in and then type in diskpart and hit Enter.
- Then list the available disks by running "list disk" and you should see your usb device.
- Select your USB drive by typing "select disk #" and hit Enter.
- Clean the partitions on the disk by typing "clean" and hit Enter.
Now create the partition by running the following command:
create partition primary
Select and format the new boot partition by running the following command:
format fs=ntfs quick
Set the partition active by typing "active" and hit Enter.
Exit Diskpart by typing Exit.
Now double click the Windows 8 ISO you downloaded to mount it in Windows 8. Browse to the ISO files in Windows Explorer and copy \sources\install.wim to the same folder as you copied Imagex.exe.
Identify the drive letter assigned to your USB drive by Windows Explorer and run the following command from the folder that contains Imagex.exe and install.wim:
imagex.exe /apply install.wim 1 d:\
(Replace d with the drive letter of your USB drive.)
Once the image has applied you need to setup the boot record on the USB Drive. At the administrative level command prompt run:
bcdboot.exe d:\windows /s d: /f ALL
(Replace d with the drive letter of your USB drive.)
After the command has completed running you are ready to use your new Windows To Go USB device.
