USB Key detection

Hello all,
I am totally new with Arduino, but I already program under C/C++ for several years by now.

I am starting a prototype with Arduino MEGA 2560.
The topic is to read some data over a RS485 connection and over a RS232 connection.
Both connection are connected to a external device.
When I got all the wanted datas, I have to create some txt file that should be wrote down on a USB key (Module serial converter to USB).
First question: I don't want to use the already installed USB connexion (security reason, I don't want people to acces to the Arduino), is this a good solution? I found some other usb device, so I'm not sure.
Second question: I have to detect if the system loose the USB key during the process and before my files are fully written, I choose the 18-19 pin, but how cans I detect that the USB key is present or lost??
Thank you for your help.

pilou007:
When I got all the wanted datas, I have to create some txt file that should be wrote down on a USB key (Module serial converter to USB).

That device is for connecting a PC to a TTL Serial port. It will not do anything with a USB block storage device (like a "Thumb Drive" or "Flash Drive"). To write a text file on a USB block storage device you will need something like a USB Host Shield connected to your Arduino.
I recommend you give up on writing files on a USB block storage device and instead write files on an SD card using the built-in SD library. To read the file on a PC you can insert the SD card into an SD card reader.

Hello,
Thanks for your answer.
Have you got some exemple of Host shield?
I got some other, could you tell me wich one is the best?
https://www.gotronic.fr/art-convertisseur-usb-serie-a000107-24008.htm
WaveShare
http://www.waveshare.com/ft232-usb-uart-board-micro.htm
http://www.waveshare.com/cp2102-usb-uart-board-micro.htm

hello,
Ok, I will give up the USB and use SD card.
How could I close this topic?