If the VPOP3 archive facility is reporting “Possible error accessing main store directory” and the archive store directory is a drive on another network computer, then there are a few possible difficulties that you have to be aware of.
If VPOP3 is running as a service, then it does not have access to mapped drives, all drives have to be referred to as UNC paths, eg \\Server\share\path
If VPOP3 is running as a service, then it could be running in a different user account from the account of the currently logged in user. In this case, you need to make sure that the account in which VPOP3 is running has the appropriate permissions to the shared network drive. This usually requires you to go to Admin Tools → Services in Windows, edit the properties of the VPOP3 service, and make it log in as a different user, then give that user permissions on the shared network drive.