MacBook Pro lost SSD during macOS High Sierra update I have a MacBook Pro 2013 with two hard drives. One of them is the original MacBook’s 500 GB HDD drive and I have extra SSD drive that I installed instead off cd rom. Mac OS is installed on both of drives but I use SSD as my system drive. 

Today I started to install the new macOS High Sierra on the SSD system drive and after the first reboot the Mac showed a progress bar that wasn’t moving for 15 minutes. So I made reset (holding power button). Now my MBP restarted from HDD and I can’t find my SSD in Disk Utility. It is hard to say how I am worried now about data stored on SSD. 

What should I do to finally boot from my SSD safely without loosing data? 

Upated: Disk Utility shows the SSD drive but it doesn’t show any logic drives here:



Also I found some restoring software that shows my drive structure:
You should know that updating to High Sierra normally means to a change in the used disk format. (Apple introduced APFS for macOS High Sierra which will be used instead of HFS+). If you now updated the system on your SSD the SSD will probably be formatted as APFS and therefore the older version of OSX won't be able to read it's content.

Did you apply any encryption on the SSD system? Because I had trouble with my system too when the update produced a unknown failure and I tried to access the system partition using an El Kapitan system. (I was forced to wipe all data and sync back my time machine backup)

Maybe try to access the data by booting in recovery mode (because the recovery system should now also be upgraded to High Sierra)