nico75,
You need to mount the Windows partition of whatever it calls first.
You find out which device name in Linux using a root terminal command
The Windows partition is always Type 7 denoting it is NTFS and should be the 1st or 2nd partition in a hard disk.
If this partition is say sda2 you have to create a mounting point in directory /mnt and mount that partition manually first, using root terminal commands
Code:
mkdir /mnt/sda2
mount /dev/sda2 /mnt/sda2
ls /mnt/sda2
The last command confirms its content.
Your Windows partition sda2 is now ready to accept the first 512 bytes from the partition /dev/sda7.
Please note dd does not need the partition to be mounted but if you are storing the output in a partition then the output destination has to be mounted first.