Hello Jonas,
I was also facing same problem in past. I followed some steps, which are not very much logical but
after following those I am able to proceed for next.
1) Prepare SD card Bootable on windows using Utility which is given by TI. Not with scripts.
2) It is required first time only.
I don't know the reason but this is my experience.
otherwise check SD card Connector pins. This problem may arise due to hardware also.
Hope this will be helpful for you,
Regards,
Vishal.