Ask the Rasp Pi3 to boot on the PIDrive disk?

On the Raspi3 (with an SD card) you have to tell the Raspbpi3 to boot on the USB drive:

sudo echo program _usb _boot _mode=1 | sudo tee -a /boot/config.txt
sudo reboot

To verify that this has worked, type:

sudo vcgencmd otp _dump | grep 17:

The answer must be:

17:3020000a

The following shall be switched off cleanly:

sudo halt

Unplug the alim, remove the SD and replug the alim.