"You may not use the file. There is not enough permission to use pst files." error while importing .pst files from the external device in Outlook 2010

Set the permission as "Everyone" on the .pst files for import

1. Right click the .pst file you are about to import and click Property. In this article, backup.pst was used for .pst file as an example.
   
   
    
2. In the backup.pst property screen, click Security, and then Edit.
   
   
    
3. In the backup.pst permission screen, click Add.
   
   
    
4. In the Select user, computer, service account or group screen, enter Everyone and then click Confirm.
   
   
    
5. In both the backup.pst permission and backup.pst property screen, click Confirm.
   
   
    
6. Import the .pst file back to see if the issue has been resolved.