Swapping Hard Drives between PS4's

So I bought a brand new PS4 Slim because my old one started having errors. I thought that I could fix it by changing the hard drive, but It did nothing. Now I have an old PS4 with a 1tb hard drive going to waste and a brand new PS4 Slim with a 500gb hard drive. Can I swap them without reformatting them? I saw online that the hard drive would reformat automatically but I'm not sure. And if you can't, how do I do this? Thanks!