The main thing to glean from this issue is “not try to recover or otherwise write data to the affected partition, nor should you format the partition”
From Microsoft – http://answers.microsoft.com/en-us/windows/forum/windows_10-files/partitions-may-be-missing-after-installing-the/ffafb34b-df6e-4c61-927d-babf29b46b87
Microsoft has received a small number of reports of partitions not appearing in File Explorer after installing the Anniversary Update. When viewing the partition in Disk Management, it appears as RAW, unallocated space.
Microsoft is aware of this issue and is working on a solution to be released through Windows Update. Any data you have stored on the affected partition is still there. We recommend that you not try to recover or otherwise write data to the affected partition, nor should you format the partition. We will update this thread when a patch is released. If you would like to be notified when this thread is updated, click the Subscribe button at the end of this post.
For up to days 10 days after installing the Anniversary Update, Windows includes the ability to “go back” or return to the previous build if you are experiencing this issue and do not want to wait for the patch to be released.
To uninstall the Anniversary Update:
- Open the Settings app.
- Select Update & security and then the Recovery tab.
- Under “Go back to an earlier build”, click the Get started button and follow the instructions.
(Note: If more than 10 days have elapsed since installing the Anniversary Update or the “Go back” option is otherwise not available; we ask for your patience until a patch can be released. Please check back on this thread for an update.)