Okay, you have opened my eyes now and the write to the USB-stick wrote only to the FAT32 partition making it FAT12, but HP gets confused when MBR says its something else...

now the only problem is if I make changes to my 256MB image, I can't get it back on there. It seems read-only :S

Okay, after filling it with 000000... in Hexplorer, the dd utility gave me access to my "new raw device" but the floppy image still didn't start - so I guess I need to make changes to the boot sector...