For some time now a Pi2B (_not_ a plus) has been booting reliably
using the "bootcode.bin" method described in 
https://www.raspberrypi.com/documentation/computers/raspberry-pi.html#special-bootcode-bin-only-boot-mode

The disk in this case was an old 40GB SATA mechanical in a USB2 enclosure,
connected via a USB3 powered hub. 

Lately a newer 1TB SATA disk in a USB3 enclosure was configured with 13-stable.
Connected alone to the hub, boot seemingly never got started, with no
output of any kind. To my surprise, if the USB2 disk was also connected,
the USB3 disk was discovered first and booted correctly, with the USB2
device recognized as /dev/da1.

Turning on the serial UART for bootcode.bin on the MicroSD card and re-trying
the USB3 device alone produced a lengthy output ending in a successful boot.
However, a reboot try stalled, with the output collected in the file named
bootlog-fail. 

Re-connecting the USB2 disk and power-cycling yielded a successful boot
from the USB3 disk. That output is in the file 
bootlog.success