Perform following steps for mentioned issues.

1. Bios settings

To adjust the BIOS Settings:

 1. In the BIOS make sure "support legacy USB" is disabled.

2. Verify that the power is on for this USB drive.

3. Verify that the USB drive is plugged into an USB port that is integrated into the motherboard before starting the computer.

2. Drive letter not being set properly:

To Mount USB drive to the proper drive letter:

1. Verify that in Windows (on the same machine) that the following drive letters are not assigned to the USB drive: A,B,C,X,Z

2. If the drive letters above are not the cause, mount the drive manually in the recovery environment per the following:

      a. From the recovery environment, Go to Analyze option on the left hand side and select the option to "Open Command Shell Window".

      b. Type "Diskpart" and hit Enter.

      c. Type "list vol"

      d. Select the USB disk by typing, "select vol #"

      e. Type Assign letter=(any letter that is free)
           A Drive Letter should be now assigned.  Type, "list vol" again to confirm that it is.

      f. Type "exit" , close CMD Window, and the volume should now be visible.
 

For additional information on diskpart refer to: http://technet.microsoft.com/en-us/library/cc757491(WS.10).aspx