USB problem - persistent enumeration failures

Hello,

I have a macbook air and use a WD external to back up to time machine.

Lately when it sleeps it appears to improperly eject the WD drive.

Now the WD doesn't show up at all when I plug it into the USB socket.

The drive does power up but isn't read by the computer.

Console reports the following

AppleUSBHostPort::disconnect: persistent enumeration failures

If I plug the WD into another computer (using the same lead) everything is fine and I have tried using a different lead on the air with no joy. I have tried plugging another USB device into the air and it works fine.

Please can you help me?

Thanks Paul