Here are the steps to mount a Samsung Galaxy SII from command line:
From the smart phone: Go to settings > Wireless and network > USB utilities.
Click on the "Connect storage to PC" icon.
Now, Plug the cable into the phone and to the computer.
You will get a new screen that says "USB Connected". Click the "Connect USB storage" icon.
You will get a new colored andoid screen showing that the USB mass storage is active.
Su or sudo to root.
mount /dev/sdb /mnt/usb
You will now be able to transfer your files back and forth from command line.
reverse out your steps when you unmount.