This is the wrong conclusion.
1 / Mega2560 i DUE is a 3V system. SD card with both Mega2560 and DUE works OK. Ethernet 28j60 or W5100 with both Mega and DUE works OK.
2/ But Ethernet does not work with the SD card at the same time. This is clearly an SPI problem. The problem for many years. Many posts on this topic. Unfortunately, Ethernet and SD card is a very desirable combination, but no post solves the problem correctly. mpcz, 19aug2019