What will happen to the files when you change your disk partitioning as changing the Windows? Just imagine that your D drive size is 100MB and entirely full. Where will the files go when you reduce its size? (Please don't joke about it)

Most likely the data would be erased and lost. My reasoning is that when you shrink a partition, that space is then formatted and partitioned into a new drive. If you are intending on shrinking a drive, be absolutely sure you have backed up ALL data as you cannot be sure what data will be affected. Best of luck to you.