I'm glad it worked @JDW! I think you're right that the ROM patches Turbo040 applies at boot is the culprit. ROM disk support like in the base BMOW ROM image also don't work with Turbo040.
Have you seen that forum member @ZaneKaminski is creating a new ROM SIMM with many of the same features as ROM-inator? Maybe Zane's new ROM code can fix HD20 support with Turbo040 sometime in the future.
The SD2SCSI (v5.1) works in all my other classic Macs as it is, without any configuration changes and with the same SD card and filesystem, so it's unlikely to be an incompatibility.
I've run out of Mac compatible SCSI drives now as they all seem to have died with age.
Try the Micronet utility. We need something that the drive performs the low-level format, and not the software itself. Think of a firmware syscall. I am not at home right now so i cant listen to the physical sound.
But Miniscribe drives are supposed to sweep the stepper motor back and forth at least once, and then begin to hunt for the negative cylinder just before cylinder/track 0. this contains the formatting and geometry information. Then it moves on. This information is highly specific to the controller that formatted the drive. the low level formatting is also specific to the controller.
if any of those tracks are unreadable for any reason, the drive's firmware will continue to hunt over and over for that track until it reads it error free.
If the error is caused by a magnetism issue, Only a low level format will fix this. If its not a magnetic issue, then either the cylinder is damaged on the platter, or the head is bad and in either of those cases there isnt much you can do.
The only way to rule anything out is a true low level format issued from a proper SCSI command to the drive itself.