if you are using the old (VFD only) i-boot without OSD it may work. Load from USB is slow in that old iboot because of USB1 drivers.
workaround is to load the kernel from the sata drive wich is much faster.
The newer OSD i-boot uses fast EHCIdriver for USB but will not start any other kernel than the Titannit kernel. only for Developers there is a setfree option wich disables the kernel check