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.
If you had a book which had on its Contents page:
and you crossed it out, then wrote:
someone looking for Chapter 1 is still going to find all the text in the right place (as long as it was less than 50 pages).
Changing the partitition table is like changing the Contents page; it doesn't mess with the rest of the data. And if the new table points to the same place it did before, the data can still be found.
That said, if the filesystem still thinks it's 1TB, you may end up with future problems unless you resize it to fit the reduced partition.