It sounds as though the external drive has become corrupted and this is why it (or one of its partitions) isn't being displayed and Disk Utility isn't loading the details. Try opening Disk Utility without the external disk connected. Once opened, plug in the external drive. If it then shows up, run First Aid on the drive to fix issues. If that still fails, you may have to erase the drive or partition, although this will destroy any data on it.
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