Generally D3DLOCK_DISCARD is described as below:
D3DLOCK_DISCARD indicates that the application does not need to keep the old vertex or index data in the buffer. If the graphics processor is still using the buffer when lock is called with D3DLOCK_DISCARD, a pointer to a new region of memory is returned instead of the old buffer data. This allows the graphics processor to continue using the old data while the application places data in the new buffer. No additional memory management is required in the application; the old buffer is reused or destroyed automatically when the graphics processor is finished with it. Note that locking a buffer with D3DLOCK_DISCARD always discards the entire buffer, specifying a nonzero offset or limited size field does not preserve information in unlocked areas of the buffer.
OK so D3DLOCK_DISCARD doesn't return data only a pointer to a new memory graphic card address which has to be fully initialized by me before unlock() is done.