So, it’s been happening for a couple of weeks now, but has deteriorated considerably, and I haven’t gotten everything off the drive yet. The symptom is that the drive will mount on boot up, stay mounted for up to a half hour and then cycle through attempts to mount again. At first, it would eventually re-mount. In fact last week it would stay mounted for up to a day at a time. This morning, it 's really hit or miss whether it will mount at all.
Re-seating the external power and USB connectors at both ends.
Removing the case, removing the interface card and re-seating the interface card.
Putting the drive in the fridge and then on top of a blue ice pack while backing up files.
There doesn’t seem to be anything wrong with the actual drive. My files are always there when it does manage to mount. It’s only the USB connection that is flaky.