Perhaps hard, as you asked one company what to get, and then got it elsewhere. The bods who sold it to you can claim 'you got what you asked for, not our fault it is not what you wanted.'
But the distance selling regs should allow you to send it back and get a refund..Over to others to go into the gory legal details.
I suggest you put it in writing ASAP that you dont want the HDD, and post it tomorrow. I assume you only got it recently...if too long ago, you might be out of luck.