Why grub2 is not starting when external HD (containing a clonezilla backup image) is plugged in?

I have a Dell XPS 9570 with dual boot setting (ubuntu alongside windows 10). Normally grub2 menu works like a charm, except a very special case; when an external HD containing a Clonezilla image is plugged in, then GRUB2 never shows up and windows boot automatically. The problem is not associated with the existence of an external HD in general, but only if this HD contains a Clonezilla image into one of its directories. Any idea why that happens?