Remove Exchange database: "The mailbox database contains one or more mailboxes..."

We have moved all the mailboxes from database "PRUEBA21" to another one because we want to delete it

This powershell with no results confirms there are no mailboxes left on that database:

[PS] D:\>Get-Mailbox -Database PRUEBA21
[PS] D:\>

Now, we delete the database on the EMC or via powershell but a warning appears:

We have no archives, no public folders. These commands allows to check it:

[PS] D:\>Get-Mailbox -Database "DATABASE" -Archive
[PS] D:\>Get-Mailbox -Database "DATABASE" -PublicFolder

In my case, the problem was there were Arbitration mailboxes:

[PS] D:\>Get-Mailbox -Database PRUEBA21 -Arbitration | fl Name

Name : SystemMailbox{bb4682c5-9341-4cb9-6ff7-d53d41dc928c}
Name : Migration.8f3e7716-2011-43e4-96b1-aba56d229136

So, creating the moves requests to another database…

[PS] D:\>Get-Mailbox -Database PRUEBA21 -Arbitration | New-MoveRequest -TargetDatabase "DB1"

DisplayName               StatusDetail              TotalMailboxSize          TotalArchiveSize         PercentComplete
-----------               ------------              ----------------          ----------------         ---------------
Microsoft Exchange        Queued                    2.562 MB (2,686,751 by...                          0
Microsoft Exchange Mig... Queued                    2.312 MB (2,424,506 by...                          0

…and waiting for them to complete…

[PS] D:\>Get-MoveRequest

DisplayName                                    Status                    TargetDatabase
-----------                                    ------                    --------------
Microsoft Exchange                             Completed                 DB1
Microsoft Exchange Migration                   Completed                 DB1

…solves the database deletion problem