I have similar issues with external USB Flash drives on Big Sur 11.0.1 (MacBook Pro (Retina, 15-inch, Mid 2015)). After plugging in USB drive I can't find it in Finder but it still can be founded under /Volumes folder. Open Finder -> Go -> Go to folder /Volumes (shortcut SHIFT + CMD + G). I hope it'll help.

Got the same problem on iPhone 11 Pro iOS 14.6 and CS-17 described solution helped! Thanks CS-17