So, I have a 16-inch MacBookPro that just started showing this problem. Big Sur 11.2.3 installed. The USB drives (yes, now two of them) will no longer auto mount. However, their locations are showing in the /Volumes directory as if they are still mounted - only not really since they are (a) not connected and (b) the folder permissions are locked. What is more interesting is that on a 15-inch MacBookPro Big Sur 11.2.3 these same exact USB drives mount correctly (as they always had). Both machines are very similarly configured as to software. Very minimal system software installed (iStats and LittleSnitch) plus the assortment of developer tools, browsers, and editors. Also, I can mount these drives via the terminal (bash shell) via "sudo diskutil mount -mountPoint /path /dev/disk2s1" In fact, I can even mount them onto the /Volumnes/diskname mount point (/Volumes/Andromeda) and have them work until I unmount them. Even in Disk Utility, I can see the volumes but not mount them - even if they were mounted and I unmount them with Disk Utility and then try to mount them again. Complete failure with: "Could not mount “Andromeda”. (com.apple.DiskManagement.disenter error 49180.)" I have rebooted, rebooted in safe mode, rebooted and reset the SMC, etc. I have also tried to remove the /Volumes/Andromeda mount point while the drive was not connected. Nothing lets me, even when booting into the recovery mode. Always claiming it is not permitted. This seems like some corruption within the OS and it is somewhere written to disk such that it does not clear up and has not cleared up over the days I have been trying to recover from this.