Installing for i386-pc platform Winusb

This is strange. I suggest you apply the 512 error fix. Maybe you have an UEFI PC and GRUB is struggling to determine what kind of bootloader to install on USB. Adding the target argument will force the installation of a MBR bootloader.

512 error fix

You must edit the winusb script which is located probably in /usr/bin (I don't have it installed so I can't tell). Open it with any text editor with root permissions (e.g. gksu gedit /usr/bin/winusb). Now search for grub-install in that file (it may be on line 401) and change that line so it looks like this:

grub-install --target=i386-pc --boot-directory="$partitionMountPath/boot" "$device"

Save the script. Install the package grub-pc-bin and try again to use WinUSB.

WinUSB only creates MBR bootable USB drives. If you want to install Windows in EFI mode a different method should be used.

A manual bootable USB creation guide can be found on my website.

If this doesn't work either, I suggest the manual method.