I'm trying to get some legacy data out of an old Movable Type v3.1 set up. I'm accessing the MySQL database directly on our web server using phpMyAdmin. The posts table is showing as having only 718 records, with the last record created in 2007. This is incorrect. I'm unfamiliar with MT's implementation but is it possible for Movable Type to store data in more than table in more than one database?
When I use the backup / export functionality in the Movable Type admin area, the full dataset is exported.
No, a single MT installation will access only one database, and all posts records are stored in the mt_entry table. The MySQL server and database name are defined in your mt-config.cgi configuration file (with the DBHost and Database directives respectively). Are you sure you're accessing the same database on the same server as what is configured for your MT installation?
(Edit: MT can be configured to use more than one DB in a master/slave configuration, but it's usually not the case and I'm unsure that possibility was existent in 3.1. In any case, the slave should have the same number of posts than the master. My best guess from what you are saying is that you're not accessing the same database or table within phpMyAdmin as MT is.)