This usually means that the USB Device option is enabled in the System BIOS' Boot Order or Boot Sequence.
Please do the following:
Additional Information: Some System BIOS' may not include the ability to disable USB devices from the Boot Order/Boot Sequence. If this is the case, it is recommended that you disconnect the USB External Drive from your system until the system is fully started into the Operating System; at which time, you can connect the USB Drive to your system.
Note: Dell and other machines have an option for "USB Emulation". Disabling this setting (with the external USB drive disconnected) has been observed to overcome this error condition.