No, John is not using RA8875, it is 5" CPLD.
The problem I gather you are already aware of in passing, SdFat/Ethernet and SPI issues. John and I have been in communication with this issue, but I suggested he contact you directly as your understanding of the SPI library for DUE is clearly much better than mine.
The problem John is having is getting sdfat and ethernet to co-exist, since UTFT_GHL uses the same DMA code used by sdfat, it stands to reason my library will cause the same fatal error. But WHAT is the problem? Are you aware of any way (with transactions or otherwise) of getting Ethernet to not die during a SdFat file write?
We have already tried digitalPin(Ethernet_CS, HIGH); before an SD operation and digitalPin(SD_CS, HIGH) before any ethernet operations, but to no avail, that I am afraid is where my SPI knowledge ends...
Hope you can shed some light on this issue!?