Hi @hibouambigu - welcome to Zynthian. You can try another cable, but yours may not be faulty, and the other cable will have the same problem if that’s the case. I don’t know if there is a way to select the slower speed for the SD card, and if you would even want that option.
If you choose to accept that you’ll live with having to put the card directly in the RPI SD slot, I do have one suggestion to avoid having to disassemble your Zynthian when you want to update from a new image. You can set the Pi’s EEPROM to boot from USB first, then the SD slot if it doesn’t find a bootable USB drive. That way you can run standard RaspiOS from USB and use it to rewrite the SD card. Details on how to set that up are in this Zynthian Pocket post, after the pictures: