Nautilus not showing mounted drives in side bar

You need to bookmark them and will likely want to rename the bookmark. This GIF illustrates the technique:

Nautilus 18 bookmark.gif

  1. Pick a partition from your "Other Locations" list.
  2. Click the upper right "hamburger" (☰) menu.
  3. From the dropdown menu click the center menu button to bookmark it.
  4. Move to the new sidebar entry and rename it from the mount name ("c" in my example) to something more meaningful ("Windows 10 SSD" in my example).