Unfortunately, Classic can get you only so far. Sure enough, it fakes out your older software fairly well—but your Mac is not actually running Mac OS 9. Anytime a piece of software tries to communicate with some physical component of your Mac, such as the SCSI, USB, FireWire, or serial ports, it will bruise its knuckles on the stainless-steel dome of Mac OS X, which is really in charge of your ports. That's why a lot of older add-on equipment, including USB-to-serial adapters, certain printers, SCSI cards, scanners, and so on may not run properly in the Classic environment.
What this kind of equipment really needs, of course, is Mac OS X–specific driver software. If drivers exist (check the manufacturer's Web site), you can once again use your gear.
Otherwise, you have only one alternative when you want your external gadgets to work properly with your Mac, just the way they did when it ran Mac OS 9: Restart the Mac in Mac OS 9.
At that point, you've returned to complete compatibility with all your old gadgets and all your old programs. When you're finished, you can restart the Mac again, this time with Mac OS X in charge. This ability to switch back and forth between two radically different operating systems on the same computer is called dual booting.