Your machine does not work with Mac OS® X Classic mode because your machine has Removable Disk related features and the USB storage driver is using the USB port.
However, you can use the Mac OS® X directly to access Removable Disk related features or restart your computer into Mac OS® 9.x.
In order to reboot the computer into OS 9, you will need to change the
Startup Disk. Please select the Startup settings from:
System Preferences-->Startup Disk-->Mac OS 9 System
And please reboot the Macintosh.