Error when trying to attch datat base , SQL 2008

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SQL-Server

SQL-Server-2008

hello all ,
i am try to attach my data base i do the required steps and choose my .mdf file and also .ldf file
but sql throw this exception

TITLE: Microsoft SQL Server Management Studio
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Attach database failed for Server 'Y3MN-PC'. (Microsoft.SqlServer.Smo)

For help, click: http://go.microsoft.com/fwlink?ProdName=Microsoft+SQL+Server&ProdVer=10.50.1447.4+((KJ_RTM).100213-0103+)&EvtSrc=Microsoft.SqlServer.Management.Smo.ExceptionTemplates.FailedOperationExceptionText&EvtID=Attach+database+Server&LinkId=20476

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ADDITIONAL INFORMATION:

An exception occurred while executing a Transact-SQL statement or batch. (Microsoft.SqlServer.ConnectionInfo)

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Unable to open the physical file "G:\XX\XX.mdf". Operating system error 5: "5(Access is denied.)". (Microsoft SQL Server, Error: 5120)

For help, click: http://go.microsoft.com/fwlink?ProdName=Microsoft+SQL+Server&ProdVer=10.00.1600&EvtSrc=MSSQLServer&EvtID=5120&LinkId=20476

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BUTTONS:

OK
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how can i solve that error ...?