Problem booting from sd-card

[giel]

Thanks for your reply! How do I do that from commandline?

Maybe its something else:
I saw in the script on https://forum.pine64.org/showthread.php?tid=4998 the following line just before the dd-command:
umount -f /dev/mmcblk1*

I didn't do that.... Maybe that's the problem?...... But is it possible to dd to an unmounted sd-card?... Apparently it is if iI read the script right.....