My Book Essentials drive not recognized under El Capitan

I have been using a 3TB MyBook Essentials external drive on my wife’s IMac as a Time Machine drive. When I booted this morning to install some updates (something she never does, so I have to attend to it every few months), I saw that this drive was not recognized. I have tried everything to try to get it to show up, but the IMac simply does not recognize that there is a drive attached to the USB port. I suspect this is a behavior under ElCapitan. Is there a solution for this? (To state the obvious, I cannot diagnose it with Utilities if the computer doesn’t even recognize a drive is there.)

Do you have another computer you can use for testing? I would also recommend replacing the USB 3.0 cable with a Micro USB 2.0 cable, which fits in the “flat” half of the connector. This will force USB 2.0 compatibility mode in case there is a USB 3.0 driver issue related to the update.