Incorrect data was entered into the Exchange Mailbox Store field and then imported into the collection. This resulted in repeated failures of the migration process. Editing the file to remove the entries made no difference as the values had been entered in to the SQL database. Every run of a migration process repopulated that column in the TSV with the incorrect data that was initially input.
Once data is added to the SQL table, some fields can not be deleted. They can however be edited to hold the correct values.
To resolve this issue run the following PowerShell cmdlets:
1- Get-mailboxdatabase xxxxx | fl distinguishedname
This will give you the path to the storage database data. It will look like this:
CN=MDB3,CN=Databases,CN=Exchange Administrative Group (FYDIBOHF23SPDLT),CN=Administrative Groups,CN=<Company Name>,CN=Microsoft Exchange,CN=Services,CN=Configuration,DC=<domain>,DC=corp
2- Replace or add the data into the Exchange Database Store column and import the collection back in to SQL.
Now when you run the migration functions they will complete successfully.
xxxxx is the target database name.