It is likely that your _ASSEMBLY folder is a Library.
Check your Project settings, and if you have to make changes to a Library, use a suitable project (the one specially set up to enable changes to Library parts, with CAUTION).
The assembly file does not belong to custom library.
If it even does I could not copy it from the beginning. As you can see I do copy of assembly and than open this copy which has accessible but not editable parts. The Standard Parts (library parts) are not accessible at all (grey).
