Well you have the ISO I think! And you have the 500GB External drive! I recommend that you:
1 Perform a Backup to save all your apps, Documents....
2 Perform a clean installation! If you have the ISO boot it into a USB or CD.
3 Restore your Laptop or hit the Refresh button in Control Panel.
4 Restore or Refresh that's up to you!
Install 10 with the 32GB space disk which will be around 28GB left you have!
5 Boot 10
6 Restore you data!
If you have doubts: Message me through the WC app!