I set my startup disk to Macintosh HD. But after opening it, the name becomes "/"
Macintosh HD is the partition name. The mount point is /, however. Other volumes are mounted in the /Volumes/ directory, such as /Volumes/MyDisk, if MyDisk is the name of the partition. This is similar to how Windows operates, where the boot disk is always C:\.
I'm not sure why Finder is showing it as /, though. On both my Mojave and Sierra machines, it shows &quot;Macintosh HD&quot;. I don't have the bottom info bar, so that may be the reason.