Mounting SSD from Monterey on Sonoma, failures

This should refer to "Sequoia" not Sonoma -- I can't change the title here (sorry) I have a 2TB SSD in a case, via USB that I want to use on my new Macbook M4. At first, it mounted fine, I could navigate the filesystem, etc. Then, there was one of those errors about not properly ejected (I've seen this before). Now, none of my Macbooks even see it -- that is, even diskutil or Disk Utility see it. While I have a separate TM disk for this system, I am a bit puzzled as to what's wrong. If I can't see the device, then I can't apply any fsck or other operations. Before I go further, I wonder if someone might offer some direction. Thanks in advance.