W7 & Clip+ : error message when connecting to PC

Using USB mode MSC, every time I connect the device to my PC I get the following error message:

There is no disk in the drive. Please insert a disk into drive\Device\Harddisk1\DR1

This eventually dissapears after several clicks on the red cross or ‘continue’ option.

ANy idea what the cause may be and how I can rectify it ?