If you want to expose a local folder on the server's hard disk as a root directory in PMA.core, you have to give the IIS user account access rights to the folder using the Windows Explorer:

Note that even though the dialog shows impersonation options, you can't use these in a local path reference context. The impersonation properties are reserved for networked content, and if you fill them in, PMA.core tries to interpret your local reference as a network path, and subsequently fails trying to access it.

So when defining local hard disk paths, make sure the impersonation options are left blank.

Root directory security overview