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You'll notice as you first start importing, that all of your media objects will jump to either "Failed", but they aren't necessarily failed if the migration hasn't finished yet. Those media objects " or "Waiting". Media objects in "Waiting" are waiting for the "second pass" (mentioned before) when all the other required items are done importing. For example: derivatives or masterfiles. During the second pass these object will move to either "Failed" or "Completed".
You can drill down to a class type and see items and some basic migration info. This is a good place to check for migration errors to work on.
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