Change the bootdisk to the hdd you want to boot from while the old one is connected. Once youre in windows format the old hdd
How exactly can you remove the OS, if you dont format it??
Is the 1.8 tb whatever it is on the same partition on this hdd, or are the files you need on another partition?
And I take it the old hdd was on this before, (not on another system)? Because you cant copy from one to the other if its from another system
The only other way youre going to do it is connect both boot change the bootdisk to the one you want to boot from.
Copy whatever from the old to the new then format the old hdd