Need to prevent EV from adding new items

For clarification, if you enabled backup mode and then a user dragged an item to the Virtual Vault, the item would be removed from the mailbox but not added to EV until the next synch, which would be after backup mode was cleared. Until then, the only copy of it would sit locally in the Vault Cache store. Leaving items in this location is not something we recommend, as they become vulnerable to a failure of the user's workstation.

You may have disabled the "Store in Vault" option on the Options tab of the Desktop Policy. This controls the ability of users to archive items manually using the Enterprise Vault toolbar on the Outlook Ribbon. However, it does not control whether users can drag items into Virtual Vault to archive them that way. That functionality is controlled by the following setting:

Desktop Policy > Advanced tab > List settings from Virtual Vault > Users can archive items

If you're looking to lock things down for a migration, you'd probably want to set all the "Users can..." options in that list to "No" for safety.

Additionally, I would not recommend disabling the Archiving Task. This will prevent users from restoring archived items to their mailboxes if they want. A couple easy ways to leave the task running but make sure it does not add any items to the Vault are:

1) Change the task's schedule to run "Never"

2) Set the task to run in report mode

Changing the required age in the Mailbox Policy will also work, but it might be tedious work depending on how many provisioning groups and policies you have. I would recommend against this just from an administrative effort perspective.

Hope that helps!

--Chris