Have you got the file viewer app called MyPad installed? If so open it and you should see two tabs, one 'local' and one 'SD1', the local is your internal tf card and should show the available free space at the bottom of the screen, and the SD1 is the external tf card (this is dependent upon the firmware you have installed). If you insert a usb flash drive it will come up as UDISK1 or 2