Grant permissions for that folder to the application pool. So you'd go to the C:FileStorage folder in Windows and edit permissions, and add IIS APPPOOLappPoolNameHere as a user, then give it full permissions.
Hope that helps.
EDIT
Step by Step Instructions...
-Open Windows Explorer
-Browse to your folder
-Right click the folder and go to Properties
-On the Security tab click Edit
-Click Add
-Under Locations, make sure it is pointing at your local machine, not a domain
-For the object name, enter below but replace MyAppPool with the name of your application pool...
IIS APPPOOLMyAppPool
-Set the permissions to Full, or just add Write, or whatever you need.
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