Pay them nothing. Apart from the (very) poor customer service the disk drive is covered for at least 12 months. Hitachi drives are normally covered for 3 years from date of purchase. I suggest you contact the store manager, provide your proof of purchase and request a full refund.
I haven`t a clue what they are talking about with this "Dynamic Mode" & "Basic Mode" - ask them to put this in writing to you. (e-Mail will do) They cannot request £19, unless you agreed to pay it in advance, to inspect a product that they supplied and is therefore within the minimum 12 month guarantee according to the Sale of Goods Act.