After you eject a DVD-RAM disc, any other disc inserted prior to restarting the computer appears to have the same amount of free space available as the first disc. This also occurs between the two sides of a dual-sided DVD-RAM disc.
Warning: To prevent the potential for data loss caused by misreporting, follow the solution described here.
Solution
Note: Upgrading to Mac OS 9.1 resolves this issue. If you already use Mac OS 9.0.x, you may download the Mac OS 9.1 Update from Apple Software Updates (http://www.apple.com/swupdates).
Otherwise, when you need to switch between DVD-RAM discs or disc sides: