Jovec's post reminded me that I had originally intended to just clone the HTPC hard drive and then upgrade on the new one. So I tried that and for some bizarre reason I absolutely cannot get my computer to boot to Windows 10 (my main PC) over the Windows 7 drive (HTPC). My main SSD is the priority boot in the BIOS, and even doing an F11 to choose boot device with it, it still defaults to the Windows 7 SSD. In fact, when I have the 2nd SSD (the one I'm trying to clone to) plugged in, it for some reason wants to default to THAT one (which has no bootable partition). I figured maybe there was something weird going on with the SATA cables, but switching them into all different configurations didn't work.
Anyway, to answer questions: With Jovec's suggestion, I cannot do that on the HTPC because it is a mini PC with only 1 ATA to plug anything in to (which is why I went with trying to use my main PC).
To answer Luthair, I have no clue. I got it from my brother-in-law. All I know is that it's an OEM key (Zotac mini PC). I would imagine it's something he had lying around, since he works in IT. I tried doing the USB install and that's when I got the error about needing an extra 6GB.
Edit: Ok, well I was able to get the drive cloned using an external enclosure, but I'm still stuck because the OS is on a 32GB partition and the Windows 10 update wants 8GB now. It pretty much ignores the remainder of the disk (I did reformat it and allocate it, however).
Edit2: I was going to try this
Clean Install Windows 10 Directly without having to Upgrade First - Windows 10 Forumsbut now I can't get the god damn USB to boot. I used the Windows Media Creation tool and it just refuses to boot. The BIOS sees it, but nada. Legacy USB on/off doesn't do shit. I'm ready to throw this fucking thing into the street.