The WHERE clause does not get populate for UPDATE statement if you fill it in using the Query Builder in the Diagram tab.
Steps to reproduce:
1. Create an UPDATE query using the Query Builder
2. In Diagram tab fill in the WHERE condition
3. Switch to Query tab, notice that the WHERE clause is not filled in and you cannot type it in either
You have to push the created query out to the Editor before execution. Type in the WHERE clause in the Editor manually.
Waiting for fix in a future release of Toad for SQL Server.