Sometime i see items in the asset store that i might want but not for the currently open project in unity. How do I download and asset but just keep it on my hard drive without putting it in the currently open project. Secondly, There are times that I DO NOT want an asset that I downloaded. But I want to save hard drive space. How do I delete a project completely off my hard drive without messing up the unity installation?
Create a new project and download the wanted asset.
For the second question - it’s a matter of opinion - so I’ll leave that open for discussion. One option is to go into the Unity directory and simply delete the asset. This may result in unwanted issues in projects you have used or imported the asset into - that will have to be cleaned up, I think each project has an instance of each asset in the project asset folder.
For windows machine this is the directory
C:\Users(yourusernamehere)\AppData\Roaming\Unity