I have a custom board with an STM32F407VGT6 and an SD card slot. I noticed that I can only access the SD card if I have a scope probe attached to the clock signal. Holding a scalpel blade between my fingers and touching the tip to the clock pad works as well.
The data signals have 33k pullups, while the clock signal is connected straight to the CPU. I did not design the board (and cannot show schematics/layout, unfortunately, but I'm happy to answer questions for further details). But I looked around and other SDIO examples don't have anything on the clock line either.
What would be the best way to fix this issue? Do I have to add capacitance?