I've resolved my problem. After disconnecting my normal sata harddisks and use their connectors on the motherboard it suddenly worked. Slax put my hitachi in mode-b and after a reboot to windows i got it to show up there aswell and the flashing went fine.
Bothered with why it didn't work in the first place, the motherboard manual learned me that i had been trying to connect it to the onboard sata raidcontroller instead of a normal sata port.
So i can safely state that the flashing will work fine with an Asus P5B and Intel ICH7 sata chipset.
Also for trial and error future notice, if your xbox light on the front starts flashing red you probably forgot to connect the video cable.
