I followed instructions to get RPi-3 to boot from a USB stick (a SanDisk Ultra 64) under updated and upgraded Raspbian Jessie. It works just fine, just as described, but won't boot when I I have a USB Logitech webcam plugged in.
1. The webcam works OK if I plug it in after the boot
2. I added "rootdelay=10" to /boot/cmdline.txt
3. I used the UUID of the /root file system in fstab instead of the /dev/sda2 designation
4. It boots OK with keyboard, mouse, and other additional memory sticks in the USB sockets
5. Using a powered USB hub to connect the camera (if a power issue) didn't help
6. Juggling what's plugged into what USB slot doesn't help either
7. Neither dmesg, syslog, nor messages show anything obvious (it probably never gets that far)
But absolutely nothing happens on the screen when a webcam is plugged in (I tried two different ones). I get one flash of the activity light on the USB boot device, but nothing thereafter. Works perfectly without the camera plugged in. Seems like the camera is fouling up the hunt for a boot device.
I'm not exactly a beginner, but neither am I an experienced systems programmer. Have appealed extensively to God (aka: Google), but alas no answer. A puzzler, and I'm stumped. Any revelations?
-- Fred P.