"AMI (American Megatrends) - AMIBIOS, AMI BIOS
•Press Del when after powering on the computer.
•Some older motherboards that use AMIBIOS may prompt for the F1 or F2 key instead."
Go to the Advanced menu
Select USB configuration
Go to USB Mass Storage Device Configuration
If it is set to hard drive you may need to change it to Auto
The above information may or may not be correct depending on the version of the BIOS you are using.
You may want to look at this page - http://www.overclockersclub.com/reviews/asus_sabertooth... - and see if it resembles your BIOS.
If not, post the version of the BIOS you are using.