One of the simplest ways around this issue is to reuse the problem part so that you can complete the save as. When the save as is done, you will be able to replace the problem part by a copy with no issue. Alternatively, you can remove the external reference before saving a copy. If those options don't work, you could try what we call an insert bar save as. Here are the steps:
- Open the assembly
- Drag the insert bar up or suppress everything in the assembly manually
- File > Save As > Save a Copy
- Resume everything in both assemblies
This method allows you to cheat the system and get around the external reference issues. However, there is danger involved. You can end up with broken references because you don't give Creo a chance to resolve them in the new assembly.