Missing partitions after installing Windows 10 Anniversary Update
Some users have reported that after upgrading their computer to Windows 10 Anniversary Update v1607, they have discovered that one or more of their disk partitions have been lost, have disappeared or are disappearing, and the disk space is displayed as unallocated raw space in disk management.
Partitions lost or missing after upgrading from Windows 10 to v1607
If you have just installed the update, you can reset your Windows 10 within 10 days via Settings > Update and settings > Restore > Back to previous version. If you have exceeded the 10-day time limit, you may have a situation ahead of you!