I have installed Debian Buster 10.2 and it works perfectly.

But before the installation process, I backed up the data of windows 10 partition like below:

With debian stretch installed I did like this:

mkdir /winstorage
mkdir /mnt
mount /dev/sdb2 /mnt
Warning error ro only.....

cp -R /mnt/* /winstorage

And then I could see all the files.

Now after the installation of Debian Buster I can't see the files in winstorage. All I see is System Volume Information and Recycle Bin. By the way /dev/sdb2 is the windows partition C: Drive. I am 100% sure. I checked it after mounting the drive.

Aren't the files being copied to winstorage? But it took 1 hour to copy all the files recursively.

Your winstorage files may have been overwritten during the upgrade. Generally speaking anything on the root partition tends to get overwritten. That's why most people have a separate home partition. If your original Windows installation is still OK, you have lost nothing but your time.

If you want to have your Windows data backed up on Buster, the method you used is fine. Just do it again. You could even set up a cron job to do it overnight from time to time.

Thank you for letting me know. Now the only option for me is to buy a dock station for my laptop's internal SSD drive which contains Windows 10. I did not want to buy that to tell you the truth. I already bought a SSD drive. And dock station is not cheap. The good one costs about sixty dollars. Anyways again thank you for your help.

If you upgraded your Debian install on the same partitions or did a new install to the same partitions, you should have created a separate partition (ntfs format) on which to put your windows files then mount it and copied. As pointed out above, it appears you just copied all the windows files to the Debian partition in the root ( / ) of the filesystem.

That's how you learn things!