|If you have filled up a 1TB drive you may be saving too much crap. That said, your bigger issue may be losing all that data when the 1TB drive fails. Do you have any backups?|
Have you tried cleaning out temp files, etc? I suggest you try using Ccleaner Slim. Get it at the link below.
To answer your original question you can use a utility designed for that purpose. Most hard drive manufacturers have a free utility to clone everything to a second installed hard drive. At least one of the drives must be made by whatever brand utility you are using.
A 1TB drive is new enough that there would be no performance advantage in cloning to another drive. As suggested above you can just add a second drive and use it for additional storage. No operating system is required on the second or subsequent hard drive/s.
After you install a second drive, move some files of the original the maintain more than 15% free space. 25% is better.