usb with arduino mega adk

how can write on a usb drive using arduino mega adk

I don't think you can - you need a USB Host shield to control the Mass USB Storage device.

I take that back, hadn't actually looked at the ADK too close:
"It has a USB host interface to connect with Android based phones, based on the MAX3421e IC."
So the hardware is there - just need the software to go with it.
I am know of libraries to access SD/uSD cards, and not USB drives.

You could try some of the links here:

doesn't mega "ADK" already have usb host sheild on it ?

Yes, see my reply #2.