By default the Service version is logged in using the "Local System" account.
This account is different to a user account you may be accessing the machine running CatTools with. It does not know about any mapped drives that are defined and are accessible by your user account.
We recommend that where possible, any Activities run by the CatTools Service that need to access a network location to read or previously stored data do so by using full UNC path names, otherwise you may find your activities report incorrect results; for example: if a backup activity always returns a status of 'Configuration is New' every time you run the activity, it is likely that the CatTools Service cannot access the network location defined in the activity setup for the current configuration file.