If you want to format your Mac's system drive, you can't do that when the Mac is running normally because the drive is in use by the operating system.

Usually, you'll want ExFAT, which is the default for the MacOS. Select the drive you want to format and then choose “Erase.”Ĥ. At the top of the window, click 'Erase.'ĥ. In the pop-up window, choose the file system you want to use.