Open Win 7's 'Control Panel' click the icon for 'Devices and Printers'. See if it's showing in there. I had a problem a few weeks back, when Win 7's 'My Computer' wasn't showing 4 of my external hard drives. But were seen in 'Device Manager' and in 'Devices and Printers'. I had to give them new letters.
Sony Xperia Z5 review: Hands-on with the phone which the Z3+ should have been