Thank you for taking the trouble to do some research. It gels with what I understand to be the guiding principle that I haven't been able to refind. The offending card was never functional in USB-mediated transfers. Although it didn't recognize the only USB device that I plugged into it (a SanDisk Cruzer 1GB), it also did not interfere with its neighbours in any obvious way.

An answer to my question about hierarchy of slot assignment in Macs is still of interest to me, but the USB-related puzzle seems to have been solved with a more appropriate marriage. (Devilish business, this miscegenation ...)

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(60x) 13DT + 3PB + PTPro; (G3) 7DT/MT; (G4) 3T + PB. System 8.1 to OS 10.5.8