Michael
First, you can add column with error details in "Save a Copy" dialogue, to get more info than just Red "X".
In 90% case when you have Red "X" next to component with "Reuse" status this means that this component has feature with external reference created in scope of original assembly. Keeping this component as "reused" (means exact same model) in a new copy of top assembly will make it "sick" - feature with external reference will be unable to properly regenerate in a new copied assembly.
So what Creo does here - it suggests (forces) you to create a new copy of this component, that will differ from the original only in that this feature with external reference will properly regenerate in new copy.
Type in new name - you will see "X" goes away.
Regards
- Vlad