ZEN Dac - unit not reconnecting when using external power supply

Hi!

I have an issue when using my Zen on my Mac. At first I only used the USB to power it, and then it would always connect to the Mac when I plugged it in - presumably because it had been unplugged.

Now I still need to unplug the external PS when I want to use the Dac again, in order for the Mac to register it.

Any thoughts?

Cheers,
Vidar