A few days of badblocks -ws doing its thing followed by a long SMART test will test the drives and take care of formatting.
Then if the old pool is raidz, follow @ChrisRJ and replicate to a new pool; if it is a mirror stripe, you may either follow @ChrisRJ anyway or replace one vdev and then remove the other from GUI (make sure you have no cloned blocks or are running a version of TrueNAS which has the vdev removal fix!)
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