To give a logical answer to your questions, I am assuming certain things.
1. You have added the HDD to another laptop which already has windows installed.
2. You want to merge the data from second HDD with first HDD in the laptop.
What I suggest to you is that, provided you have enough free space on the laptop's original HDD, you can copy all the data files like DOC, XLS, PDF, TXT, JPG, BMP, and other user created files from the second hard disk to the first hard disk. Don't copy any OS related files like files from Windows, Program Files folders to the first hard disk.
Once this is over, you can remove the HDD and put it back in the old laptop.
Or if you want to keep this HDD permanently in the new laptop, format it and clean out so that the old OS is removed from it. Now you can use this hard disk as a data storage to your new laptop.