My headless Orange Pi Zero Plus was configured to boot from a powered external USB2 drive with 3 partitions, a 48Gb linux partition populated by armbian-config, a 4Gb swap partition and a 800Gb NTFS data partition (shared by Samba).

The setup is stable and works beautifully when powered up from the powered off state.
The Pi fails to start when I reboot it.  I suspect that the USB port or drive or both need some kind of reset when a reboot occurs.

Has anybody encountered this problem?   Is there a known solution?

Kernel for H5/A64 is (labeled as) experimental because there are many such small troubles. I am not aware of this particular one and dunno if it is already solved. You can try to upgrade to latest nightly DEV kernel/u-boot (from armbian-config) to see if this problem was fixed ... but this is not recommended for any kind of productive deployment. If you can afford to experiment and you know how to rescue the system in case it will not boot, then proceed, otherwise ... wait for next tested update. 

Thanks
If it's not a known issue, then I will open it up and connect into the debug uart and do some debugging when I can find some time.