I'm also having the same issue after upgrading my Late 2014 iMac Retina 27inch to Big Sur. I have a 17TB Drobo 5D, connected to my iMac via Thunderbolt, and it doesn't mount normally. The Drobo drive still exists and been recognized both on Drobo Dashboards and Disk Utility App, but it simple doesn't Mount. After re-connecting the Drobo Drive via USB, it worked and mounted normally, so it's obvious that this is a Thunderbolt trouble. Drobo also announced this issue officially on their website.