SQL iteration-Insertion plus renaming
I'm not a DBA/DBMS guru, so just some ideas:


Turn off indexes on tData2 during the migration.
Do it in smaller chunks for smaller transaction log. (Batch commit on large INSERT operation in native SQL?, How to implement LIMIT with Microsoft SQL Server?)
Make sure that [tData].[Key] has an index.
I'd consider replacing the subselect with a join if it's possible at all.