USB Accessories Disabled

I have a new Mac mini (specs below) with an Apple USB-C to Lightening cable. When I try to connect my iPad Air 2 to the computer to sync, I get an error message saying:

USB Accessories Disabled

Unplug the accessory using too much power to re-enable USB devices. [Close]

All other USB devices (2 Western Digital external hard drives) remain connected and accessible despite the error message that seems to indicate they would be disconnected.

I disconnected the external hard drives, shut down, unplugged the computer for 10 minutes, then restarted the computer to no effect.

APPLE: How am I supposed to sync my iPad Air 2 to my latest-generation Mac mini?

Mac mini specs:

  Model Name: Mac mini

  Model Identifier: Macmini8,1

  Processor Name: 6-Core Intel Core i7

  Processor Speed: 3.2 GHz

  Number of Processors: 1

  Total Number of Cores: 6

  L2 Cache (per Core): 256 KB

  L3 Cache: 12 MB

  Hyper-Threading Technology: Enabled

  Memory: 32 GB

  Boot ROM Version: 1037.80.53.0.0 (iBridge: 17.16.13050.0.0,0)

  Activation Lock Status: Enabled

UPDATE/CORRECTION: What I typed in italic font above isn't accurate. I had unmounted but not disconnected the external hard drives. When I unplugged the drives and rebooted I was able to sync my iPad without the error.

Nevertheless, the error message above wasn't very helpful. It indicates some USB device is drawing too much power when the problem is that there were too many USB devices attached and the aggregate power draw is too much. A more helpful message might be,

"Insufficient power for connected USB devices. Unplug any devices not in use and try again."