I have the same issue. External USB drive's that still mount in Catalina don't mount in already-running Big Sur. But if they're plugged in when Big Sur starts up, they'll mount OK.
AlanWaller
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Well, please disregard my post of a few hours ago. I shut down, started up, and now external USB drives are mounting fine. Beats me. Sorry if I'm in anyway out of line ... I'm new here so thanks in advance for slack.