ApplicationPoolIdentity.Managed Service Account (ie a pseudo identity) that you won't find in the local usrs of the computer.App_Data folder is not automatically configured to allow write access for ApplicationPoolIdentity. Therefore the attempt to write to the App_Data folder will fail.
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## Process ##
Either create a unique AppPool identity and grant it rights, or stick with the ApplicationPoolIdentity and grant it rights to the App_Data folder: ApplicationPoolIdentity
## Resources ##
http://support.microsoft.com/kb/2005172