A simple registry edit turns your WP7 device into a USB drive.
Open the Registry Editor (regedit), go to HKEY_LOCAL_MACHINE\SYSTEM and then expand the CurrentControlSet\Enum\USB folder.
Search for PortableDeviceNameSpaceExcludeFromShell and there you will get the other setting which are need to set are below:
> Change ShowInShell from 0 to 1.
> Change PortableDeviceNameSpaceExcludeFromShell from 1 to 0.
> Change EnableLegacySupport from 0 to 1. That's it. If there's more than one Windows Phone 7 device listed.
Cheers! Now all done.
Note: Remember to take backup before doing any registry changes..