Q:

"This list cannot be deleted because one or more lists are related to it." - Which?

A user and myself can't delete a library due to something still using it.
How can i find out what that something is? Can the library be deleted while keeping whatever the something is using?

A:

Open the library in Internet Explorer.
Click a checkbox in its list.
Click the Library tab in the Library Tools tab at the top.
Edit Library (this will open the library in Sharepoint Designer).
Uncheck "Display this list on the Quick Launch" if you only need it gone there. Save.
Edit list columns and delete all lookup columns. Save and ignore value range warning.
Delete the library.

I guess nothing depended on it but itself.

