No dual boot option on Windows 7 desktop install of Zorin Core

Installing Zorin OS
1
  1. Installed Try version of Core (Nvidia drivers) from USB flash drive onto Intel i5 desktop running Windows 7 Home Premium. Worked fine
  2. So decided to install dual boot version via external WP Passport 2TB drive.
  3. Installation to external disk worked fine to a point, but then crashed at the end (sorry no screens).
  4. When trying to re-install, no dual boot option appears. Install doesn't see Windows drives at all. Can only install a full wipe Core from USB flash drive now.

Has Zorin installed something permanent on my Windows machine (boot grub?) that is preventing a dual boot option from appearing?

Windows loads and runs fine normally.