…except that once I ejected it, it would not mount again from the same USB port…. Mounted fine from a different port. Hoping just a ‘time since dismounted’ issue.

Fixed my Samsung T3 SSD connection like this: open disc utility select the drive in left side of window (mine showed in a different section below other discs that were working). click the “first aid” icon at top of window choose “Run” in the pop up window. mine ran and fixed, now visible in finder, all files in tact.

Same problem - and since my time machine backup is on an external drive, how can i even revert to an OS that works? Terrible experience with this “update” - not an update if it makes things you rely on useless, @apple.