Hi Thomas

Since totally purging the transaction log is the key step of this fix, i do not think you are going to be able to do this without taking mirroring down.

You will need to restart the instance anyway as part of the steps so you will need downtime anyway. My suggestion would be the following (you will need to test this thoroughly as its only a best guess.)

1. Take databases out of mirroring and keep traffic pointed at primary.

2. Backup the database and the log of the primary and then restore to the secondary.

3. Before putting the secondary back into mirroring follow the steps to remove TDE but do not remove the Key of the certificate.

4. Put the databases back into mirroring and then manually failover

5. Repeat steps 1 - 3 on the secondary (This was the primary until you failed over)

6. Put the databases back into mirroring and then failback if needed.

Is i said, the it just my best guess so please excuse me if there are some glaring oversights. It goes without saying that you will need to test this before running on production, while you are doing this ensure that you remove the certificate and restart the test instance after you have followed steps 1 - 6 to confirm that TDE has been removed.

Hope that helps.

Steve