Just to add to this: place the inline resistance (try 20 - 200 Ohms) at the driving side of the signal lines.
If you imagine the wire as a capacitor, you want this inline resistance to limit the current that this capacitor is charged with in order to reduce the ringing.
Thanks for the explanation, @recursinging. For my application, higher speeds aren’t needed, what I really need is more flash. After getting my test code running, I added FatFS to my real code and was 5KB over limit. Guess I need to go back to trying to figure out why my QSPI-based patch storage scheme very occasionally wipes clean. 
Keep an eye on this PR, when it’s merged, there will be more program space available.
Until then, if you’re close to the 128k limit, try compiling with -0s - it might reduce the size just enough.
The slower speed and the addition of some 20 Ohm resistors seems to have remedied the problem for me, as well. Thanks!
Hi everyone.
I had no issue with my Pod and no issue with my breadboard too!
The troubles came when I added the SD card reader with daisy on a protoboard. Changing the GPIO speed did the trick.
I should try to add some 20R on the way too!
Hello all…
I just wanted to report that as of this post, January 2023, the above mentioned changes regarding SDIO speed set to low appear to still be necessary to make a seed board use the Adafruit SDIO SDMicro breakout card also mentioned in this thread.