A moment ago I unmounted my 1TB HDD with 400GB of content and I partition it into two different partitions, obviously keeping the space that was already occupied. I did because I don't care if the content get corrupted, but after I did it everything is still working perfectly, when I thought everything would be corrupted.

I am possibly a complete ignorant on this subject, but due to the nature of the HDD and how it writes and reads data I expected it to corrupt everything, why didn't it happen? On an SSD on the other hand I would not consider that possible because it is not even a mechanical part where the information is stored.