SQLDriverConnect fails when attempting to connect to Failover_Partner
When database mirroring fails over to the mirror machine, the server runs startup code on the database including recovery steps to ensure the consistency of transactions in the database.  This process can take some time; perhaps on the order of 10 to 20 seconds.

You need to wait for the database on dby to come online.  I would recommend retrying the connection several times, with a wait of a second or two in between.

Don't disable connection pooling, that will only cause you performance problems, and has nothing to do with database mirroring.