Here are two bootlogs of the Pi2B with a single USB3 HDD connected.
The microSD card has bootcode.bin and timeout files in the DOS partition.
The line 
program_usb_boot_timeout=1
is present in config.txt on the USB3 dos partition.

No changes were made between attempts. The machine had just finished 
a build/install cycle for world and kernel. It was rebooted with
shutdown -r now
and the first try succeeded, to the extent that it reached the loader.

Since it got that far, I stopped the loader and issued a reboot command.

The result is recorded in bootlog.fail. The boot process just stopped,
so far as I can tell. If there's an escape sequence it would really help.

As a last step, in u-boot, usb_pgood_delay was set to 10000. That did
seem to slow the boot process, but didn't make it any more reliable 
and I didn't capture those console logs.

In at least a few cases, the P2B very quickly started trying to netboot.

Finally, I removed the USB3 disk entirely, replaced it with the old
USB2 disk. The machine booted successfully, hands-off.