MacBook "loses" disk

Running Sierra - following a reboot, one of two internal drives no longer shows up. It shows up in Disk Utility in Recovery mode - First Aid checks it out as ok. In normal mode, or safe mode, it doesn't show up in Finder and Disk Utility can't load info on any drives.........Tried removing DiskUtility.plist in Preferences. No difference.

All Mail and Spotlight indexing also lost.......

Will probably have to reload OS - which will solve it until the next forced reboot.

Any pointers appreciated. Thank you.