Nautilus file manager shows the root directory as File System for your Ubuntu system partition. If you have other partitions (or volumes), it shows them as xx GB Filesystem if they’re not named or labelled.
Using Disk Utility is one of the effective ways to name a partition easily:
– go to System > Administration > Disk Utility
– select the item Hard Disk.
In the Volumes section, click a partition you want to label then:
– click “Edit Filesystem Label” (Note 1)
In the Label box, enter a name, e.g. Data-Disk, and click Apply.
The file manager should now show the partition label, such as Data-Disk, instead of xx GB Filesystem.

Note 1: If the option for “Edit Filesystem Label” is not shown, click “Unmount Volume” before hand. In case you can’t unmount a volume, try Storage Device Manager to unmount it. See Auto Mount Drives at System Startup.

Note 2: This tip is for naming a partition using Disk Utility, use other advanced features such as format, edit or delete partition with caution as they can delete data on your disk.