Sad … found several additional issues … I used the sd card reader on an cheap 2.8" TFT Board (china) … This has an 3.3v regulator on board caused a lot of stability issues … so I bridged the regulator … then it has 3x1k resistors in series in the spi lines… in several other schematics I only find them in pull up configuration or non at all. So I bridged them … still the mbed example works and the ported version to platfom io not.
I ordered myself a STM32f746 discovery board because it fits better for my project (TFT, Touch, Ethernet, SD card + internal flash chip - huge sd-ram … for 50€ not a bad deal. See how far I get with this one. Hope to avoid hardware issues with this “all in one” soluition