Hardrive that is tampered with and now they blamin

I bought an External Hardrive on the 28/02/2015 and I got to my car to test if it was working and it was I then took it back to the store 10mins from the time it was purchased I was then advised that the unit cant be exchanged and that the unit was tampered with I then advised them that I will wait for them to get back to me as they advised that it will be on the 02/03/2015 they then got back to me saying that they not exchanging me or refunding me as the supplier wont give them a credit I told them that this is not my problem as I bought a unit and expected it to work the Manager Angelo Sheldon advised that he wont be assisting me as it cant be the staff so it must be me I still disagree with that as I was only made aware that the incorrect unit is in the box when the technician came to me after he opened the unit in store to show me it is the wrong unit. so I am asking myself why did they open the unit in the store if it was suppose to go to the supplier this means that the staff of Hifi is opening units in the store before they send it away to the supplier and who says in the time that the technician in the store didn't swop the unit then come to me and say it was tampered with.