Hi, I had the same problem of my Nexus 7 (2012) suddenly stopping to appear as a disk drive but now as an MTP Device. This meant I couldn't copy/paste data as I could previously. After trying n trying I finally found the root of the problem. While using CCleaner to clean the registry, this had also modified the wpdusb.sys file (one of the default drivers provided by Microsoft). This in no was detracts from the usefulness of CCleaner as registry mods need special care anyway! This file WAS there [/WINDOWS/Sys32/Drivers] but the size was just 18 KB. On checking online, I found out that this file should be around the 38 KB mark. Simple fix - System Restore to an earlier point and voila! You can also download this file separately and install it to replace the errant earlier one.
Wpdusb.sys is essential to have access to USB-connected MTP devices. Ensure this as well as winusb.sys is present (35 KB).
* Also ensure your PC has up-to-date drivers for the Nexus
* Note, some of the USB cables don't support data transfer, so try as far as possible to use the orig cable that came with the Nexus. It def. works!
* Ensure that the Connection type on the tab is set to MTP and not Camera & that USB Debugging is off (if you don't see Developer Options in your tab Settings, it probably is off!).
If this helps you get back on track, don't forget to leave a smile as a reply - it might help someone else! Peace out....