I know for a fact that about 200 BTC has been lost... A few friends of mine started mining back when mining was REALLY good. And well one, lets call him JOE, reformatted and reinstalled his computer, forgetting to backup his wallet. Then the other friend, lets call him BOB, had to leave for military training. Well BOB asked JOE to house-sit for him. BOB told JOE that he had backed up everything and that JOE was free to reformat/reinstall his computer. Well BOB didn't backup either. JOE had something like 70 BTC and BOB had around 140 BTC. So I wonder how much of that 6.534 mil. has been lost?!?