I've also been having problems reading an SD card on the MKR Eth shield on a MKR Zero. The card works on the MKR Zero's slot, but not on the shield's slot. The chip select is changed to point to the desired slot. This works when using a Mega2560 and Ethernet Shield 2. I think the issue is the MKR zero has two SPI busses, one of which is dedicated to the SD card, and the other one that is brought out to the header pins. The SPI library and Ethernet library do not seem to account for this. Setting the enable pin for the ethernet controller HIGH to avoid bus contention does not help, so something is missing in dealing with the two SPI busses. Any hints appreciated.