There's also the possibility that the drive alignment is so far out of whack that you're not seeing the data on the SCP disks. This is unlikely in drives that haven't been diddled with, but unless you bought the drive NOS, you don't know that.
Grab a known-good 8" floppy--the format doesn't really matter, as long as it's fairly IBM 3740 compliant (Single-side, single-density, 128 byte FM sectors). An RX01 floppy is good for that. See what IMD shows. --Chuck