Mount-SPContentDatabase : Exception of type 'Microsoft.SharePoint.Upgrade.SPUpgradeException' was thrown. At D:TempAttach_OldContentDBs_DoIt.ps1:63 char:28 + Mount-SPContentDatabase <<<< -WebApplication $webapp -Name $thisdb + CategoryInfo : InvalidData: (Microsoft.Share...ContentDatabase:SPCmdletMountContentDatabase) [Mount-SPContentDatabase], SPUpg radeException + FullyQualifiedErrorId : Microsoft.SharePoint.PowerShell.SPCmdletMountContentDatabase
Have a look into the SQL Server Management Studio. – The content database is read only. That’s the problem