I was having this problem in Big Sur as well. 2 of my 3 external drives would not show up on the desktop or in disk utility. If I booted in Catalina they all showed up. What fixed it for me was swapping out a USB cable. I was using a long (10ft) cable and I replaced it with a 2ft cable and now all the drives show in Big Sur. It seems, in my case, the Big Sur didn't like the long cable...
nitsua
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