Restore a specific file group to a test environment without restoring the Primary file group (Error 3154)
Well, sad to say you can't do a piecemeal restore and bring a secondary filegroup online without the primary being online first. If there is a hack to get around this I am not aware of it.

If your primary filegroup is at 1TB and you are not using it, why not just do a one time shrink to a more manageable size? This is not uncommon to do in production environments where a large amount of data may have been purged and you want to reclaim the space. As well, if you have no ability to test your backups your are in a far worse situation.