I would deal as follows with the old OS partition on the HDD - all operations from the new OS on the HDD.
1. Copy all the user data (Documents, Music, Videos, Pictures, Downloads) from that partition to the external disk. And do not forget to make a backup of your Favorites in Internet Explorer or whatever browser you use. Make a seperate folder for all that stuff on your external disk so that it does not get mixed up with other files.
2. Once you are sure that everything that is worth keeping is backed up, go into Disk Management and delete that partition and the volume until you have unallocated space. Then create a new partition in that unallocated space.
3. Now copy the saved folders Documents, Music, etc. to that new partiton. The downloads you copy into the download folder of the SSD. The Favorites you import to the Internet Explorer.
4. Rename the 4 folders on the new HDD partition to e.g. Old Documents, Old Music, Old Videos, Old Pictures. I named mine Whs Documents, Whs Music, etc. So you choose what you like. You will see later why this is good to do.
5. Now right click on each of those renamed folders and INCLUDE them into the approriate library - Documents to the documents library, Music to the music library, etc.
6. Now you will have this situation in your libraries - note my folders on the HDD start with Whs
