I have been having the same problems not just with big sur but with Catalina also I have lost a lot of important work and file and more than one flash drive which has cost me a lot. but what is bothering me is how Apple have not said anything, they keep bringing out new versions before making sure they are fit for use. they need to work on the issues that are happening before moving forward. i have spend thousands on there Products only for them to let me down and cost me more. this is a joke. Apple are proving to be unreliable.