Problem with new windows install on new Sata m.2

[ActuallyGoose]

So I tried installing windows onto my m.2 SSD (data). and now when I turn my computer on, it asks which windows I want to use, either my old or my new one. The new one being on my m.2 SSD, and the old one being on my SATA SSD. But after about a second of being on that screen, it automatically logs into the new one on my m.2 and I'm left unable to get rid of the windows install on my old SATA SSD. I've tried using Samsung magician on my old SSD (It's an 850 EVO btw). But it's telling me that it "failed to create bootable USB device". So, as I haven't really messed around with wiping or erasing SSD's before. I'm just unsure of what the best way to remove everything is, as most of the windows files are "protected" and I'm unable to manually delete everything, without spending a lot of time manually giving myself permissions.

Sorry this is so long and typed somewhat complicated, But I'm unsure of how to explain my situation as someone who doesn't know a lot about this area.

But thank you to anyone who can help me out in advance.