Although Mac OS 9.1 has been tested and approved by Digidesign for use with Mbox, some users have reported system performance issues, such as lower track and plugin count. For some users, updating to Mac OS 9.2.2 has improved their system performance with Mbox.
We are looking into why some users are able to use OS 9.1 without problems while others are not. Possible reasons could be an altered OS 9.1 System file or software added by programs installed after OS 9.1.
If you experience abnormal plugin or other system performance issues with Mbox and OS 9.1, Digidesign recommends that you update to Mac OS 9.2.2, which will replace the System file and update USB and other Apple extensions. Mac OS 9.2.2 is a free update from OS 9, available from apple.com or by using the Software Update Control Panel.