Are you on the latest version of Mozy? Do you have the drive selected for a local backup? I heard the older versions will black list the drive you select for your local backup (so, if you select the drive for a local backup, it will disappear in the file system tab). However, I believe that is not true for the latest version.
If that isn't the issue, you may want to ask your question in the mozy community for a little more exposure (community.mozy.com).