Thanks for the responses, guys. I figured out my problem. The used mac originally came with OS 9.0.4 and when it didn't work with any USB card I installed, I assumed the best option was to update it to the last version of OS 9 and work on the problem in that environment.
According to some information I stumbled on, 9.2.2 can have USB-related problems with some Macs without built-in USB ports. I was totally unaware of this, so trying to fix the problem by upgrading from 9.0.4 up to 9.2.2 was apparently my first mistake.
I reinstalled the OS a few times, but I always upgraded to 9.2.2 before troubleshooting the USB issue, thinking this would give me the best chance to fix the problem. Ultimately, I just needed a fresh install of OS 9.04, with the USB extensions, and no further updates, except for USB Card Support 1.4.1.
According to some information I stumbled on, 9.2.2 can have USB-related problems with some Macs without built-in USB ports. I was totally unaware of this, so trying to fix the problem by upgrading from 9.0.4 up to 9.2.2 was apparently my first mistake.
I reinstalled the OS a few times, but I always upgraded to 9.2.2 before troubleshooting the USB issue, thinking this would give me the best chance to fix the problem. Ultimately, I just needed a fresh install of OS 9.04, with the USB extensions, and no further updates, except for USB Card Support 1.4.1.