Someone

        /* Copy the data. */
        uint64_t uOffset = 0;
        uint64_t cbRemaining = cbSize;
        
        do
        {
            size_t cbThisRead = RT_MIN(VD_BUFFER_SIZE, cbRemaining);

            rc = vdReadHelper(pDiskFrom, pImageFrom, uOffset, pvBuf,
                              cbThisRead);
            if (VBOX_FAILURE(rc))
                break;

            rc = vdWriteHelper(pDiskTo, pImageTo, uOffset, pvBuf,
                               cbThisRead);
            if (VBOX_FAILURE(rc))
                break;

            uOffset += cbThisRead;
            cbRemaining -= cbThisRead;
        } while (uOffset < cbSize);