Hi,
The model object must actually be used somewhere. I would suggest it is most likely used in something you thought you copied but accidently changed the original too. It happens to me frequently then I have to go and change the original items back to the original settings. e.g. you thought you copied "Dim1" and were editing "Copy of Dim1" but you actually changed the original "Dim1" first. It is so easy to get tied up.
You can;t jus tremove the link. I'm afraid you will have to find where the model object is being used and change it back to the original item. Then you will be able to get rid of your object once all references to it are removed.
If you are brave enough you might find it easier to to look for references to your model object directly in the database C:\ProgramData\Autodesk\Advance Steel 2017\User2\Data\AstorAddin.mdb. You could simply edit the entry and remove the "Copy of ". That is not for the faint hearted and easy to screw up so back it up first!!!. Please don't blame me if it all goes wrong 
Rule No.1 when editing Drawing Styles:- Use extreme caution and back up frequently.
I hope that helps.
Aleck Giles, Structures Consultant, Graitec
Did you find this post helpful? Feel free to Like this post.
Did your question get successfully answered? Then click on the ACCEPT SOLUTION button.

LinkedIn