Windows Phone 7 could work as a USB mass storage but its feature wasn’t enabled by default. It needs some tricks to switch USB mass storage feature on such as registry hacking , WP7 jailbreak tool and engage a little unsupported modifications. Those ways were able to drag and drop files in Windows Explorer. But those ways were unfeasible for many of Windows Phone 7 users.
Now, we already have easier method to enable USB mass storage feature on WP7. No dirty works like stated on above paragraph. What we should to do is just download the small program, called as WP7 USB Storage Enabler. It will help us much where the tool works on both locked and unlocked devices. But please notice requirement to enable USB mass storage, the tool needs the .Net 2.0 framework and Zune software. But surely you have already the both programs since they already installed when you sync the WP7 to PC.
After all prerequisites have been satisfied, just launch the WP7 USB mass enabler than click the enable button. It will enabling the USB mass storage feature, if you already finish, just back to the tool than click disable button.