So I though that loading files would be a trivial thing. So I implemented some basic ListView-like visual control for the OSD and started debugging it. Strange.. suddenly there are sdcard errors all over the place. Sometimes they are immediately after the start, sometimes later. But most frequently they appear after invoking the OSD. And if I invoke the OSD automatically at the boot time, the errors increase when the program executed by the emulator seems to create a heavier load on the emulator.

I tried a ton of regular things you do when trying to weed out a software error. All my stacks were silly oversized. I juggled around initialisation routines. Moved things between tasks and played with task priorities. Even sacrificed proper screen updates, giving SD card task the highest priority. But alas:

Searching for this kind of stuff also does rarely give useful input because every time it's someone forgot to connect a wire on a breadboard or something. But one of the discussions that I found mentioned power issues. 0x109 and CRC errors tend to be symptomatic to poor power delivery.

I examined the board and found buggerall capacitors:

There are some near the buck-boost converter near the USB-C connector and there's exactly 1 (one) 100nF capacitor near the ESP32 module. That's all. I suddenly remembered how this board sometimes doesn't want to boot, or wants me to plug it to another port on the hub, or seems to boot but then enters some weird loop and you need to replug it again...

Since yesterday I'm a happy owner of a brand new optical stereo microscope which I was eager to try out. Here's what I added:

220 tantalum and 10uF ceramic to the right of R15:

10uF ceramic near the ESP32 module. It has internal caps but it doesn't hurt having more:

All together:

Yes, it did fix the problem. SD card reads, no random inexplicable errors.

Update:
although it does seem to help, it's not ideal anyway. There are still errors. They seem to appear less frequently when I power this board with a powerbank rather than USB hub.

Update 2:

SD card problems persist and I'm not saying that they can only be explained by aliens, but it's aliens. I don't know if the capacitors helped anything at all now. Stuff is janky as usual.