that either

It's not the HDD (swapping it results in the same problem) and not specifically the display (though it could be the entire video card, as none of the three external display outputs works either), so it'll possibly be dropped off at a shop for investigation (with the blank hard drive inside it), whilst this cable can be used in conjunction with an old desktop from the loft, an old keyboard from the cupboard above the stairs and the television's RGB input socket to see if the populated HDD is happy switching hardware for a little while.