snatch1eB
As it was said, you may try unRAID which would handle even the different size drives.
As alternative, it looks like mdadm should fit your requirements. You can use smth like openmediavault for that.
I would definitely contact seller for that. Warranty is one of the things why you get the new drive, at least that is critical to me. Return the drive if they would refuse to provide you with the warranty.
It depends on your use case, if the idea is simply to get the shared storage, the windows option should work there.
I would avoid Storage Spaces since it is too unreliable, especially the parity option. As alternative to it you might use stablebit drivepool with snapraid or collect the drives into software raid inside of linux vm. Shouldn’t be an issue with Starwinds cvm https://www.starwindsoftware.com/blog/file-share-with-starwind-vsan
i don’t have space for disk image by the way.
Get the backup drive for it, make the image and restore after. It’s the most efficient way.
Look for the best deals and the best $/TB ratio. Not really much other advices rather than having more backup copies next time.
You can use FreeFileSync to sync the data across two drives. I wouldn’t say that it’s a complete backup, but should be enough for your use case.