Grey screen when booting from original disk, doing Internet recovery or booting from High Sierra boot disk I own a MacBook Pro from 2012 that pops a gray scren everytime I try to boot.

When I try to boot from my original disk, the screen is gray with a question mark folder. I tried Internet recovery and it seems to work out. When the loading bar is full, a screen with Apple logo appears for like 10 seconds, and then a blank grey screen, nothing happens even if I wait for a while.

I then tried to boot on Linux mint with a live key to test the hardware, it fails to install, and the screen remains blank for a while as well, same for any other Linux distribution.

Lastly, I tried to boot from a High Sierra disk that I created using a friends MacBook. The screen asks me to install it, I do so, and after about 80% of the loading bar, the screen turns grey again...

It does look like a hardware problem, but I can't manage to load hardware test by holding down D key.

What is the reason for this behaviour? How can I resolve this issue?
The flashing question mark means that it could not find a volume to boot from. This indicates either that the volume is corrupt, disk not working or an error in the SATA controller that talks to the disk (or all of the above). The grey screen I believe comes from the stuck storage device enumeration right before the OS X Utilities window should show.

I've encountered the exact same behaviour on a MacBook Pro of similar age and it in my case it turned out to be a failure on the internal hard drive controller. I'd suggest you open it up and disconnect the hard drive, then try starting from Internet Recovery, the diagnostics and running from an external storage (such as live CD or USB stick). It may be that only your hard drive failed so a drive replacement could be a complete fix.

It is quite easy to open the computer and temporarily disconnect the drive, see the detailed procedure at iFixit