How to mount USB Drive in Single User Mode in Mac OS X
In order to run un-signed Kernel extensions in Mac, you need to
disable Apple Signature check for loading Kernel extensions.

Reboot Mac in Recovery mode(Command + R), then select the menu item "Utilities / Terminal", then run the command

csrutil disable


Reboot in Single User mode (Command + S)
Enter the following commands,

fsck –fy

mount –uw /

Create a directory to mount USB Drive

mkdir /Volumes/usbmnt

Before plugging in the USB Drive, get the list of "disk" devices available by running the command

ls -l /dev/disk*

Plug in the USB disk
Identify the new device entry for the USB disk by again running the command

ls -l /dev/disk*


Usually the disk will be mounted like '/dev/disk2s1'
Identify the file system of the USB disk

fstyp /dev/disk2s1

Based on the file system format, mount the USB Drive; my USB format was "FAT", so I used msdos:

mount_msdos /dev/disk2s1 /Volumes/usbmnt



That's it; now the USB drive is mounted.  Use the contents of the drive at the path filesystem "/Volumes/usbmnt".