esx 5.1 usb lost

hi all. strange behaviour on my esx5.1

my win2k3 vm server has an RDX backup solution (ArcServe D2D) which works fine... I have an issue now..i loose my usb connection to the local RDX drive...

When i try to re-add the usb controller it's ok...but when i want to add the usb peripheral i see "not available"

any clue?

thanks