Mac OS is based on Unix.
Drives are not mounted in the root folder(s) - which will typically be "/".
They are usually mounted in /dev/.
To ensure where your drives are mounted, open Terminal and type:
diskutil list
Use that to get your mounting points.
For instance (ugly code!):
FileSystemView fsv = FileSystemView.getFileSystemView();
File dev = fsv.getChild(fsv.getRoots()[0], "dev");
for (String listed: dev.list()) {
System.out.println(listed);
}