Question:

There is an unloaded overlay xref but it is not able to detach it from the host drawing.

Warning at bottom of Xref Manager says: "Unable to detach. Multiple reference detected"

But it doesn't nest in other xrefs in the Tree View.

Solution:

Select the file in Xref Manager (CLASSICXREF command),

Click Browse button and change the file to any other valid drawing,

Click OK to close the Xref Manager,

Locate the reference in the drawing,

Open Xref Manager again, select the xref and click Bind ->Insert,

Click OK to close the Xref Manager,

Delete the reference block and purge the drawing.