Now that my theory is broken I came up with a new theory
Most SLC cards are Fixed Disk mode. That is: they support UDMA during true IDE mode.
There is an utility to convert Sandisk Extreme to Fixed mode. May try that, but the utility is old and doesn't run on Win10.
fixed drive = IDE mode
removable drive = pcmcia mode
that might explain people's success using SLC as most, if not all, slc run on ide mode with udma support.