Why did this happen?

Thanks for your note. I had a Chat session with Apple Support last night. He had me do some kind of total restart of the system. Then I plugged the WD drive into the port that had not recognized the drive, and this time it did. So I’m hoping the problem is solved–though I don’t still don’t understand what caused it. The tech guy said something about maybe corrupted port, but that doesn’t really make sense to me. Plugging in the WD drive was the first device plugged in there.