Yeah with newer revisions it boots like the rpi 3, so you don't even need an sd card.
I also have an rpi 3 which boots from USB without problems with multiple devices attached. But my rpi 2b won't

I don't think the fstab will matter. It fails even before reaching the kernel or initrd. The version is 1.1 I also did some more tests. I can boot from USB with other USB devices attached when I put the whole fat partition on the sd card. So I guess it won't be something power related. I also see so...

I used the new bootcode.bin to boot my rpi 2 from USB. i.e. SD card has a fat partition with bootcode.bin and the USB stick has a fat partition with the rest of the boot files and an ext4 partition with the rootfs. This works but only as long as I don't have any other USB device plugged in. If I plu...