You need to plugin the USB, and read the doc and code.

Dependencies:   USBHost mbed

Fork of USBHostMSD_HelloWorld by Samuel Mokrani

Revision:
6:a801dcc9ff68
Parent:
5:fc580cab6a61
Child:
7:f5c5fad802c3
--- a/USBHost.lib	Wed Mar 13 15:50:06 2013 +0000
+++ b/USBHost.lib	Wed Mar 13 17:34:04 2013 +0000
@@ -1,1 +1,1 @@
-https://mbed.org/users/mbed_official/code/USBHost/#1571e517a91b
+https://mbed.org/users/mbed_official/code/USBHost/#e7e63ac7ae5f