Never upgraded much had same pc for 10+ years. I added a ssd not a nvme because motherboard so old. I copied my hdd to my ssd. Now when I download new games the ssd is still filled up is it okay to format the ssd.
Will it still run the applications for example diablo 2 installed.
If i take off the hdd and only use the ssd it doesn't find the file for example diablo 2. I don't want to use all the space on the ssd I want to use hdd as space. Is it okay to format the ssd is my question.
Will it run diablo 2 from the hdd or I still gotta reinstall it?
Is there some kind of balance in installing on hdd and using ssd?

You need to install Diablo 2 on the SSD or else it’ll run on your old hard drives slower speeds. You have to go into the battle net launcher and specify where you want to install it. Your SSD should be the D: drive if it doesn’t have your OS on it

If your diablo 2 is on your hdd, you can copy it to your ssd. It's usually in . You just need to right click on the folder, hit copy, then paste it in some location on the ssd. You don't have to reinstall the game. Just look for Diablo II or Diablo II Resurrected depending on which version you are running.