The OTP is set, the Pi 3 boots from a USB stick, all working fine.
I have cloned the USB stick to a 64 GB drive with a SATA adaptor with split USB/power connectors. The Pi does not even blink after it would usually boot from the stick.
Bought a WD green 240GB and Onrico enclosure. It has a USB3 connector; plugged into the Pi USB, power from a 3A power supply. The WD pulls no more than 200mA (as per specs up to 2.2A, which the Pi cannot provide); which is not the issue while loading (checked current with scope). The USB drive comes on after 5 seconds, which might be too long for the Pi boot sequence.
I read a few posts on the forum, one points out a specific SATA chip ASMEDIA (or so) for this to work. Is this the general consensus?
Anything else I could/should try?