With Windows Storages Spaces you can add two different hard drives that act like one. No RAID. But I'm pretty sure you have to format them so not an option here.

I'm not sure about regular copy tools. I use TeraCopy too but I think it doesn't preserve file attributes. I'd recommend to use a cloning tool, go to bed and check next day.

If your 8TB drive is external I'd buy another 8TB. Like others said it's too much risk having a 16TB. If it's internal and you can't add another HDD go for the 16TB. But a very big internal drive (always on) is even more risky.

 

Thanks for all the feedback, guys. This one has me intrigued. So with the StarTech, would I be able to just pop my current drive in there and then pop in another drive and windows would detect the entire thing as 1 drive?

 

What you are describing is a RAID setup using two or more HDD,SSD or NMVe drives operating as one.
Good for storage. Not active audio programs or video editing.