In Exchange 2010 and 2013 you can exclude mailbox databases from automatic mailbox provisioning load balancing which is used by Exchange to automatically distribute mailboxes across all available mailbox databases in an evenly manner.
Today while I was working at for one customer migrating from Exchange 2010 to 2013 while I was moving a mailbox I received the Unable to move mailbox database is excluded from provisioning error:
New-MoveRequest -Identity mig-test -TargetDatabase DB01EX2K13 The database DB01EX2K13 is excluded from provisioning. Please select a different target database for the move. + CategoryInfo : InvalidArgument: (mig-test:MailboxOrMailUserIdParameter) [New-MoveRequest], DatabaseExclude...ioningException + FullyQualifiedErrorId : [Server=SRV-EX2K13MB01,RequestId=8d7288d1-36bf-40b5-80bd-b03896e2072f,TimeStamp=21.10.2014 17:42:05] [FailureCategory=Cmdlet-DatabaseExcludedFromProvisioningException] 1F2EFD21,Microsoft.Exchange.Management.RecipientTasks.NewMoveRequest + PSComputerName: SRV-EX2K13MB01.helocheck.com
Under normal circumstances a manual mailbox move should ignore the excluded from provisioning but Microsoft confirmed there is a bug with Exchange 2013 which should be fixed in a future release, I personally reproduced this in my home lab with both Exchange 2013 SP1 CU 5 and CU6 but maybe other versions are affected as well.
To solve and bypass the database is excluded from provisioning error you can either disable the exclusion from provisioning all together or suspend it temporarily with one of the following cmdlets:
Set-MailboxDatabase DB01EX2K13 -IsExcludedFromProvisioning:$false Set-MailboxDatabase DB01EX2K13 -IsSuspendedFromProvisioning:$true
Once you have used one of the commands you will be able to move the mailbox without issues.