Ok, I had this same issue and found a fix. However, it's ONLY a fix if you do not care about what is on the external HDD. Open Disk Utility and hit Partition. Change it to APFS (Encrypted). I hit "Apply" and it warned me that all data would be deleted. I said ok. It wiped it and then it automatically mounted and all is well now. Mine was previously on APFS (case sensitive) and I had all of the same issues you all have had and none of the suggested fixes did not work.