A faulty product sold to me

On Friday 6th November 2015 I visited the store to purchase a decent and functional 2nd hand external hard drive. I was assisted by an energetic sales man who then fetched the device from the back office, he quickly slotted it into a laptop at the store and only showed me that the capacity of the device was available (650 g) , but he didn't play a movie nor did he play any music from it. I was convinced that the device functioned well like he told me so I then paid R500 of my hard earned money for it, I took it home and only started loading movies on Sunday but the movies wouldn't directly play from the external. I realized that this could be a serious problem and took it back for a query on Monday 9th Nov, I was then assisted by a different salesmen at the store as the initial seller had taken a day off work. I repeatedly Drove back to Stoneridge till I found the person responsible, he advised me that he would call me back on Friday 13th Nov with feedback and a possible solution. The salesman has failed to do so resulting in me having to call in daily only to find no help from him. I have made it vocal that I wanted to speak to the store manager but my request has been ignored