I have 2 TFS Servers, one running TFS 2012 and another 2013. I have to move one collection from the 2012 to the 2013 version, but when I backup and restore it in the appropriate SQL server and click on the Attach Collection button, the TFS says that it can't find any databases (project collections) to attach.
What steps do I need to take to upgrade this database to the newer version without interfering with the continuous work of the server?
On
An attach upgrade is not recommended and is far more risky than an instance upgrade. You should upgrade your TFS 2012 to TFS 2013 first and then move the collection by detaching it first. Without the detach the database week not be listed.
You can, accepting the risk, detach the 2012 database from TFS, then backup and restore the detached 2012. When you attach the system will perform an upgrade.
You have to detach the collection from TFS2012, backup the SQL database from SQL Server of TFS2012, restore the database to SQL Server of TFS2013 then attach the collection on TFS2013.