@salamisami
Are you starting flasher, before attaching the N9 ?
If not, please do so (flasher -i)

Also, make sure you are using flasher version 3.12 (the harmattan flasher)
It is also possible, besides the USB cable, that either the USB port on the PC is defect, or the USB port on the N9 itself.
Try more cables, another USB port, another PC.