Why can't I delete my custom dashboards?
One of the main reasons why you will not be allowed to delete a custom dashboard, even if you are a Super Admin, is that it is being used as a Report scheduler. Check that first and if it is not the case reach out to support for additional aid.
What does "Move Data Grid" do on my custom dashboard?
This option will move all top performers to the bottom of the page to allow them to be exported to show the full results. This is recommended when you will be using top performers to show more rows than appear in the widget with its size/location on the custom dashboard on screen.
Why don't Custom Dashboards show on LeftNav?
The custom dashboards that do show in the left-hand navigation are the ones that are directly linked to the Data Profile you are assigned to
If you are an admin user you will be able to all the custom dashboards by clicking into the manage dashboards menu