There's an interesting trend in theoretical physics that suggests subjectivity is the only source of randomness in the Universe. According to that idea, everything is deterministic but since you can't know exactly where you are in the (multi)verse, what will happen to you next depends exactly on how many versions of you exist in the (multi)verse, and in how many different versions of you they can diverge to. Kind of when you consider a coin on a table with your eyes closed. Even if there is nothing random in the side it shows, at least for all the other people looking at it, until you open your eyes you don't know which side it is. So unless you have additional information, you have to consider it as if it was random : fifty percent heads, fifty percent tail.

This is oddly relevant to mind uploading. Let me explain.

There is a version of this theory that relies on what Max Tegmark calls the "level III" multiverse. In this version, whenever someone measures a quantum process, she enters a quantum superposition made of several "relative state" that can not interact with each other and therefore each of them think they are the unique successor of who the observer was before the experiment. In other words, for each versions of the observer, not only the other versions are merely theoretical concepts with not much reality, but even if they do exist, they are merely "copies".

And to me that's how mind uploading is related to the many worlds interpretation of quantum mechanics : in this interpretation, "mind duplication", which entails most of the metaphysical issues related to mind uploading, happens all the time! And if someone is willing to suppose that whatever process producing consciousness can be reproduced by a computer with perfect fidelity, then the mind uploading procedure becomes similar to other mind duplication procedures like for instance those implied by variants of teleportation or hemispherectomy.

So to me the success of mind uploading is a subjective concept and as such it is probabilistic. If you upload your mind into N computers, then you have 1 chance out of N to experience the failure of the upload, in the sense that you will remain in your biological body. The reason is that after the upload, among all conscious entities that are willing to identify themselves as who you are, that is for instance as they have the same personality and share the same memories of the events before the upload, N of them are inside a computer and only one is in an organic body, namely the original. And if you're willing to make the fidelity assumption mentioned above, there is no reason to privilege any of these consciousness from the others.

The only way to make sure no consciousness will ever experience the failure of the upload is to suspend the consciousness before the upload procedure and resume only the versions running on a computer.