Arbitration mailboxes are where messages awaiting moderator approval are stored, the OAB is generated, and various other administrative tasks. You can not uninstall 2010 while these are still on the database.
When you try to remove the first DB in Exchange 2010, there are a few arbitration mailboxes that will prevent database deletion. You have the choice of moving, removing, or mail-disable these mailboxes. Since you cannot see these in the Exchange Management Console, you need to launch Exchange Management Shell (EMS)
This will list the arbitration mailboxes. To narrow it down to a specific database, you can edit this to:
Get-Mailbox -Arbitration -Database DB1
Once you have your get- command returning the correct list of mailboxes, it's time to move, disable or remove them. Disabling the last arbitration mailbox is not allowed, so I recommend moving them as the first preference here.
Move: Get-Mailbox -Arbitration -Database db1 | New-MoveRequest -TargetDatabase db2
Disable: Get-Mailbox -Arbitration -Database db1 | Disable-Mailbox -Arbitration
Remove: Get-Mailbox -Arbitration -Database db1 | Remove-Mailbox -Arbitration -RemoveLastArbitrationMailboxAllowed
Once you have these taken care of you should be good to go. if something breaks when you remove these Exchange will recreate them automatically if you rerun
from the installation media.
Hope this helps someone!