thx for the fast reply, but the d90 should already be in ptp-mode! found no way to switch between usb and ptp.
from another forum:
Here is the response from Nikon. "Thank you for emailing us. Again we apologize but the D90 does not act as a mass storage device while connected to your computer as the only transfer modes available are PTP/MTP. Because the camera does not act as a Mass Storage Mode you will not be able to view the camera the same way you are able to with the D80"
safety is just a superstition...