I just realized that my Adaptec 2940UW adapter has been running with the Ultra mode DISABLED, making the max transfer rate only 20MB/Sec. The drive has been working fine for almost a year like this (what was I thinking). Anyway...
I enabled the Ultra mode (40MB/Sec), and now, the drive (ST34371) doesn't work right, and I get errors reported when Windows boots, and it barely runs..freezing, unfreezing, like it's having negotiation problems.
I went back into EZ-SCSI(CTRL-A) and disabled the Ultra mode (ouch), and now it works again. The drive appears to be terminated properly.
The only other thing on the bus is a SCSI scanner (UMAX S-12), connected to the original 6' cable, with the adaptec 68-50pin adapter that terminates the unused pins. Also, the scanner does have the terminator plug installed. I thought I had everything right. The internal cable is the original one that came with the drive (very short).