External Windows bootable drive freezes boot screen
After the BSOD your mac should restart to "Start Windows Normally" screen hit Enter To Start Windows Normally
Go to device manager (Press Start and type devmgmt.msc and hit enter) and uninstall "unknown device" under "Universal Serial Bus Controllers" Tick "Delete the Driver Software for this device."
Now go to your Bootcamp folder i.e. BOOTCAMP_5.1.5621 or BOOTCAMP_5.1.5640 and go to Bootcamp > Drivers > Intel
Now run the file IntelxHCISetup.exe. It will show you the intel usb3.0 setup assistant. do not install or cancel the setup as it will delete the temporary folder in temp
Now go to Start and type taskmgr.exe and hit enter. It will open up Task Manager.
In that you have to select Intel Installation Framework and type Shift + fn + f10 to open right click menu (Right click may not work  unless you finish installing bootcamp fully).
Select "Go To Process", and again press the Shift + fn + f10 key combination to bring up the right click menu
And then select Open File Location, Now copy the contents of the folder to a new location i.e. by creating a new folder in the desktop or any other location
After copying the contents of the folder to a new location go to Device manager and right click the Unknown Device and press the Right click key combination Shift + fn + f10 and select Update Driver Software > Browse my computer for driver software and browse for the folder that you created in Step 8 and click next. It will detect the driver and installs it automatically.


That should solve your problem with the USB Drive.