Well for starters Windows 10 is actually a complete rewrite and is actually lighter on resources than 8.1 so it sounds like you have some other issue with your system. You will also only have support for 8.1 for a few more years.
Moving on. The easiest way to fix your issue is going to be to disconnect the 500GB drive and reinstall Windows in the 1TB drive. While it is possible to create those partitions manually on your main drive it isn't very easy and if you're not technical I wouldn't recommend trying.