I've found out that this is a known bug:
Despite not showing up in the nautilus bookmark pane, as described here, you can still quickly access bookmarks by right-clicking on the nautilus icon and bookmarks will be displayed as a quicklist:
But I've figured out a workaround. Would Nautilus show my bookmarks if I set my partition to be mounted at Ubuntu startup time? Yes, it would. :-)
Beware that Ubuntu mounts partitions at
/media/yourusername/partitionname when you mount them by clicking on them. So, your bookmarks will point to this location. When I tested mounting my partition using
udisks command, it mounted at
/media/partitionname. Searching has brought this solution:
How can I change the mount point path while using udisks?
Resume: Set your partition to be mounted at startup, at
Startup Applications on Ubuntu, using this command:
udisksctl mount --block-device /dev/sdaY (change Y to your partition number!)
Now my partition's bookmarks are displayed correctly.
For the ones who don't know how to add a command to be executed at startup time, I recommend reading Ubuntu documentation on Auto Mount Partitions, section Per-User Mounts, subsection Adding to startup. Section Finding the device name of your Partition explains how to find your device name (the
/dev/sdaY in my example above).