I've been given a scrapped DVR, it's an SMT C7100 and I was intending to salvage the disk drive. Inside the box it is completely clean except for the copper side of the circuit board. There are has some dark brown marks which are probably solder flux which are not of any concern to me. However there are also brown smudges, as though someone has wiped the board with a rag soaked in liquid mud. I have no idea what this could be. Could it be a chemical or biological health hazard?