I am not able to save datat on Machintosh HD as it is showing as Read only. How to change it to Read and Write both

Save your data anywhere in your home folder or in /Users/Shared.

You don't.

Just use your home folder.

You can access it with the Go menu in Finder (cmd-shift-h).

By default, its subfolders show in the SideBar, Desktop, Documents, Downloads.

You can drag favorite folders into the SideBar, or you can add some of the standard items using Finder Preferences > Sidebar.

If you open /Users by going to Computer (Go > Computer), double-Click Macintosh HD, double-click Users, you can then hold down cmd+option and drag the home (your username) to the Desktop. When you do that, it will create an Alias (shortcut). You'll see that it is creating an alias as the icon you are dragging will show a curly arrow in the corner. If it doesn't show that, you are not making an alias, so don't drop it there.

You can also drag the folder into your Dock.

If you create folders of files directly inside the home folder, they will be readable by other users on that Mac, so you should use the subfolders.

Use care when dragging a folder to the SideBar as you don't want to drop it into another folder as it will move the folder into that folder, not put it in the SideBar.

When moving a folder (or file) to the SideBar, it should look like this:

That will add a shortcut to the D0G folder in between Documents and Downloads.

That is the system drive. You don’t save data to it. Save you data to the “Data” drive.