I'm having this same issue with my Macbook Pro (2019). I've tired restarting. I've tried repairing disk but it will not mount. I'm not sure what the cause is. I have my music library on it so I don't want to format it and lose it. It's currently in Fat32. I'm unsure why Big Sur won't read it as my computer was able to read it on Catalina when I first got it.