There's a good thread on this here:
http://forum.ppcgeeks.com/showthread...highlight=wmdc
For me, I have discovered that every time I flash a new ROM, I have to switch to a different USB port. I can go to one I've used previously, but not the most recent one if that makes sense.
It probably doesn't...it doesn't even make sense to me. Let me give an example.
I use USB port A on Monday. Everything's working great. Tuesday morning, I come in, plug my phone into USB port A and then flash a new ROM. Once I'm through, I need to switch to USB port B because USB port A stops working for me. When I do, everything will work fine. Wednesday morning, I come in, plug my phone into USB port B, and realize I did something wrong on Tuesday, so I flash a new ROM again. After the flash, USB port B won't work for me anymore, but USB port A will once again work. So, I switch back to USB port A and I'm happy.
Don't ask me why this is working for me. I haven't a clue, and don't really have enough curiosity about this phenomenon to try to figure it out. Someday MS will get WMDC stable enough that I won't have to do this, I hope.
Oh, and I have much better luck using USB ports directly attached to the machine...no hubs.