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Right... of course you need the SSD! it'll change the way your machine performs drastically.
What version of macOS are you on?
Why was the HDD partitioned?
Whats on each?
What are they called?The easiest thing for you to do would be to download macOS from the App Store and then run the installer, but selecting your SSD for the instal not your current HDD's. Once installed you will be able to access your HDD but you boot from your SSD (this is an option selected in "Start up disk").
To satisfy my OCD I'd wipe then HDD and then restore what you need onto the SSD and set up the HDD as a single partitioned storage drive - unless there was a good reason to have 2 partitions?
Sweet! it's 525 GB. The HD is I think 2TB, but was partitioned by the shop I got it from in to two 1TB guys. It's very possible I don't need this extra SSD at all, as one of the 1tb guys is showing 770GB of space, the other one is maxed out though. Have backed up what I need I think. As you can tell I'm not a computer guy, this stuff burns my brain. What's the next logical step?