>Attaching may get it up a running, but in runs in a compatibility mode as the DB is not migrated to the new 2012 format.
>
>>Also, as alternative, did you try to detach this DB in SQL 2008 R2 Express, copy to another place, attach back in SQL 2008 R2 (from original location) and attach in SQL 2012 from new location?

Hmm, interesting point, I need to check compatibility mode for all my DBs as some I attached, some restored.

UPDATE. Found several DBs with lower compatibility_level, so I am going to bump it up.