Seems the problem has been identified then, USB HD! That 9 min time you gave is definitely 8x recording on the 3500!
Have you burned a DVD from your internal HD to one of those Aritas yet just to see if you get similar times to the Imation discs?
Also, is your external HD plugged into a USB 2.0 port built into your mobo or do you use a USB 2.0 PCI card? Because if you're using a PCI card you just need to make sure you installed USB 2.0 drivers if windows didn't already detect the card and install the drivers.
Just trying to see if you can still use the external HD.