$RootDirectory
gives the root directory of your file system.
Examples
Wolfram Research (2003), $RootDirectory, Wolfram Language function, https://reference.wolfram.com/language/ref/$RootDirectory.html.
Text
Wolfram Research (2003), $RootDirectory, Wolfram Language function, https://reference.wolfram.com/language/ref/$RootDirectory.html.
CMS
Wolfram Language. 2003. "$RootDirectory." Wolfram Language & System Documentation Center. Wolfram Research. https://reference.wolfram.com/language/ref/$RootDirectory.html.
APA
Wolfram Language. (2003). $RootDirectory. Wolfram Language & System Documentation Center. Retrieved from https://reference.wolfram.com/language/ref/$RootDirectory.html