I have not seen this issue before with my Color Classic. I can use any vintage SCSI drive from 40MB to 160MB: IBM, Connor, Quantum with or without external SCSI devices. The Color classic has no special SCSI termination requirements.
I'm wondering, have you recapped the logic board of your CC ?
A last resort what you can try is take a working drive and do a universal install (=minimal system for every mac ) of Mac OS 7.1 on the drive in one of your other Compact Macs. Then install the drive in the Color Classic.
If that doesn't work, you either have a faulty logic board or damaged/bad contacts in the flat cable assembly.
Given an external SCSI drive works well it are likely damaged contacts in the flat cable assembly.
There's a small possibility it's a faulty internal SCSI cable.
I'm wondering, have you recapped the logic board of your CC ?
A last resort what you can try is take a working drive and do a universal install (=minimal system for every mac ) of Mac OS 7.1 on the drive in one of your other Compact Macs. Then install the drive in the Color Classic.
If that doesn't work, you either have a faulty logic board or damaged/bad contacts in the flat cable assembly.
Given an external SCSI drive works well it are likely damaged contacts in the flat cable assembly.
There's a small possibility it's a faulty internal SCSI cable.