The number of free tapes in the Tape Library IBM TotalStorage 3573 Tape Library is less than or equals the threshold value of 2. You must add tape to the library and mark it as free in order to prevent future backups from failing. (ID: 3305)Expired tapes that are being managed by that DPM server can be reused automatically without being marked as free by the administrator. However, this alert do not take in to account the expired tapes, so you may see this alert even if the total available (free and expired) tapes are higher than the threshold value.
This is not a threshold that OpsMgr controls, it's a threshold stored within the DPM database. Currently DPM do not have any way to change this threshold. Although unsupported, you can use SQL Server Management Studio to alter the threshold value, like this:
Select the DPM database within SQL Server Management Studio and then create a New Query, type in and execute this update query (this is valid for DPM 2012):
UPDATE [dbo].[tbl_MM_MediaPool] SET [MinThresholdForFreePool] = 1 WHERE ([Name] = 'Free Media Pool') GO
For a detailed explanation on how to mark a tape as free, look at this TechNet topic.