Fixed it. I had a USB thumb drive do the same thing, my 2nd machine wouldn't recognize it in a file explorer, so I experimented with it. All it took to fix both the unrecognized USB thumb drive and the USB portable drive was to assign both a drive letter. They then both began to show up in Explorer and Unreal Commander. Thanks for pointing me to diskmgmt.msc. I have used it before, but I use it so rarely, I didn't think about it.