I had to leave it all for a while because I thought there was something obvious I was missing, hence not being able to make the necessary changes.
I just removed the card from my device, inserted it into the card reader on my computer, and you're right, I didn't lose the files. I am sorry for losing my temper though.
But what am I supposed to do when I want to send files to my device? Or should I just forget about it and use the SD card reader on my computer and transfer the files manually? I thought calibre was supposed to be able to do that.
Why am I having so much trouble with it?