I had exactly the same problem. It seems also that rootdelay hasn't worked on 14.0 or 14.1. It last worked on 13.37.
The only alternative is to create an initrd.gz or rebuild the kernel.
initrd.gz is the quickest option if you have an already working system
This worked for me
mkinitrd -c -k 3.10.17-smp -m usb-storage:ehci-pci:ehci-platform -u -w 10 -f ext4 -r /dev/sdb1
The resultant initrd.gz needs to be moved to the boot directory of your USB disk.
See also -