I have a similar issue. I’m using an iMac (late 2014) and recently upgraded to Big Sur. For a while Big Sur displayed the external drives in Finder, but now they are gone. I have a Seagate 4 TB External drive with a photo library on it so I do not want to reformat the drive. The external drive shows up in the disk utility but cannot be mounted. My old MacBook Air can recognize and read/write the Seagate drive fine. Need help.
Tprobe
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