On three different bare metal systems, trying to boot https://kojipkgs.fedoraproject.org/work/tasks/1413/12051413/Fedora-Live-KDE-x86_64-rawhide-20151204.iso from a USB stick, I hit the same problem. Quite early in boot a WARNING traceback occurs just after some errors about the USB device. Boot then fails: the system sits apparently idle for a while, but in fact it's just dracut timing out, eventually it dumps you at a dracut emergency console with an error that /dev/mapper/live-rw does not exist.
Created attachment 1102485 [details] screenshot of the trace (and errors)
Proposing as an Alpha blocker: "Release-blocking live and dedicated installer images must boot when written to optical media of an appropriate size (if applicable) and when written to a USB stick with at least one of the officially supported methods." - https://fedoraproject.org/wiki/Fedora_24_Alpha_Release_Criteria#Release-blocking_images_must_boot (in a footnote). We may need to test on a few more systems and with different writing methods, but this looks a lot like a general failure.
Confirmed that this does not happen when booting from a DVD (same image).
Note: I also see this WARNING when I boot from an optical disc then plug in a USB stick, and the attached USB stick's partitions do not show up under /dev - so I see /dev/sdb, but no /dev/sdb1 etc.
Bisected and asked upstream, https://git.kernel.org/cgit/linux/kernel/git/mkp/scsi.git/commit/?h=4.5/scsi-queue&id=221255aee67ec1c752001080aafec0c4e9390d95 fixed the issue on my local test system. Dropped the patch into rawhide.
Sorry I didn't confirm the fix for this yet, I'll get to it tomorrow. Thanks a lot for the prompt response!
As KDE lives have been broken for a while, I confirmed this bug also existed in the 2015-12-04 Xfce live, and then confirmed that the 2015-12-14 Xfce live works fine. All good, thanks again.