Am I presumed guilty, since it's clear there's "evidence" to show that I was the source of the now widely spread copy? Do I now have to prove to a judge that I actually didn't upload that file to TPB?
Guilty until proven innocent?
In other words, if your personally marked copy of a work was found floating around on the Internet and you got sued for copyright infringement, you'd probably have to convince the court that it was more likely that someone had stolen the work from you and redistributed it than that you had redistributed it in some other way yourself.
This isn't a completely safe position even then, because if someone has suffered substantial damages as a result of the work being distributed (and copyright law is infamous for inflated claims of damage caused by infringement) and you have potentially already admitted to being negligent in protecting your copy of the work then you might just find yourself jumping from the frying pan to the fire.