USB HOST problem during array write

Hi
I want to send an array of data to a USB pen drive using LPC1768.
and USBhostlite code is available here:
>ics.nxp.com/.../
and I removed Uart from codes so that it just copies data to usb.
the codes that i modified are:

void Main_Write(void){
        USB_INT32S  fdw;
        USB_INT32U  bytes_read;
        char  temp[8];
        USB_INT08U  temp_buf[504]={0};
        int i,j={0};
        bytes_read=9;
        fdw = FILE_Open(FILENAME_W, RDWR);
        if (fdw>0) {
                        do{
                                sprintf(temp,"%d",bytes_read);
                                for (j=0;j<9;j++){
                                        temp_buf [i]=(temp[j]);
                                        i++;
                                }
                                temp_buf [i]=0xA;
                                i++;
                                bytes_read ++;
                        }while(i<=504);
                        FILE_Write(fdw,temp_buf,504);
                        FILE_Close(fdw);
                }else{
                        return;
                }
}

sprintf converts bytes_read to ascii codes then it is sent to temp_buf using a do{...}while() then an "MSWRITE.TXT" file is created in usb disk and finally the buffer will be written to the usb disk.

these codes work properly, when size of the buffer(temp_buf) is less than "90". but when i change the buffer size to a bigger number(eg: 504), the program doesn't work!
it even doesn't creates any empty text file(due to file_open)
i traced file_open routine and i found that the problem is in MS_BulkRecv routine. because the program runs before calling MS_BulkRecv and halts during calling this line. this routine is as follows:

***********************************************************************************************
*                                         RECEIVE THE BULK DATA
*
* Description: This function is used to receive the bulk data
*
* Arguments  : None
*
* Returns    : OK                             if Success
*              ERR_INVALID_BOOTSIG    if Failed
*
**********************************************************************************************/
 
USB_INT32S  MS_BulkRecv (          USB_INT32U   block_number,
                                   USB_INT16U   num_blocks,
                         volatile  USB_INT08U  *user_buffer)
{
    USB_INT32S  rc;
    int i;
    volatile USB_INT08U *c = user_buffer;
    for (i=0;i<MS_BlkSize*num_blocks;i++)
        *c++ = 0;
    Fill_MSCommand(block_number, MS_BlkSize, num_blocks, MS_DATA_DIR_IN, SCSI_CMD_READ_10, 10);
    rc = Host_ProcessTD(EDBulkOut, TD_OUT, TDBuffer, CBW_SIZE);
    if (rc == OK) {
        rc = Host_ProcessTD(EDBulkIn, TD_IN, user_buffer, MS_BlkSize * num_blocks);
        if (rc == OK) {
                rc = Host_ProcessTD(EDBulkIn, TD_IN, TDBuffer, CSW_SIZE);
            if (rc == OK) {
                if (TDBuffer[12] != 0) {
                    rc = ERR_MS_CMD_FAILED;
                }
            }
        }
    }
    return (rc);
}

can any body tell me why this increase in buffer size causes the program not to work? and help me to solve it?

thanks
Amir