High Sierra won't install

Hi,

So recently I've decided to download High Sierra from El Capitan ()MacBook Pro model: Mid 2012). Since this is a pretty old model with a old HDD, I decided to try this new system out. So at first I downloaded and waited a bit for it to install - naturally it took it's time and it got to the phase where the Macbook restarts. The Mac booted up but the install was taking ages to do so I decided to switch the laptop off and try the next day. However this seemed to do some serious damage to my HDD because when I tried to load up my Mac it wouldn't go past the bootup (the loading bar would take ages to load then just switch off). I decided to go on Disk Utility to fix the HDD but I kept getting this error message (something about unmount).

Since the HDD was old I was going to change it to an SSD anyways. All of my stuff was backed up so it was fine. I changed the HDD to an SSD - this SSD i used last year for another Macbook (I only used it for a few days for work). The replacement of the HDD to the SSD was exceptional - there were no issues at hand. I booted up the Mac and started using my laptop for a few hours. Since this had El Capitan, I decided to go for High Sierra once again.

The same process of the installation happened again, however I left home during the boot up phase and the laptop died when I was out (I unplugged the charger before I left because the battery was full). I tried booting up the laptop once I plugged in the charger and was waiting for the installation to continue, however i was introduced to a folder icon with a "?". I tried booting up in recovery mode and i tried repairing the SSD this time round but I still got the error message about un mounting as I did before.

This was frustrating so I called Apple Support and the guy walked me through completely erasing the Disk and re-installing the OS. The OS has been "installing" for over 12 hours now- it says "8 minutes remaining" and has been stuck like that throughout the night.

What is going on?

Sorry for the long thread.

Thank you.