I have Mac Os sierra and I am upgrading to Mac Os High Sierra. The problem is the upgrading is not workinfg properly. Always it reaches 43% the system says that cannot install the OS and show the option to restar or change the startup disk. When I chose the last one, no startup disks appears on the list. So I quited and use command R to acces the utilities tools. Before check if the disks are okay I access the "Reinstall Mac os" option but after starts the upgrading (8 minutes remaining) it shows a error message: A server with the specified hostname could not be found.". I don'tknow more what to do. If I reboot the machine I always return to the utilities tools. It's the first time I get an error during the upgrading since I acquire my mac mini 2012 (more than 4 years). Please some help! Edited: now I am using the Safari browser from the utilities tools waiting for an answer to fix this issue. Is it possible to recover the sierra instalation without use the time machine? I forgot to create a backup today and I will lost everything I did. If not, I can install the new os in another partition to not lost the files I have stored in the documment folder and in the desktop of the Sierra instalation?