Moving a level to new folder or renaming

Hi,

I’m trying to move some levels to a new folder (but this also happens when I try to rename them). When moving it I get the error message:


Can’t save A:/Projects/Code/Unreal/RealmWars/Content/Maps/Map4_SymMountains.umap: Graph is linked to private object(s) in an external package.
External Object(s):
Texture2D_0_BaseColor (GIBakedBaseColorMaterial)
Try to find the chain of references to that object (may take some time)?

(press retry/cancel, and get this message)

The asset ‘/Game/Maps/Map4_SymMountains’ (Map4_SymMountains.umap) failed to save.
Cancel: Stop saving all assets and return to the editor.
Retry: Attempt to save the asset again.
Continue: Skip saving this asset only.

(the map then disappears from content browser and I have to refetch it from version control)

Any ideas on how to get around this?
Thanks