Success: I resolved this exact problem. When trying to install High Sierra, my Mac was stuck the black startup screen, stalling at either 90% or 100% completion between restarts.
I managed to get High Sierra installed without losing any of my files. Here is how I did it.
Failures:
- Resetting a billion times.
- Resetting PRAM.
- Resetting CMD.
- Using Safe Mode.
- Using Recovery Mode (It would freeze upon attempting to re-install the OS, and Disk Utility couldn't detect my drive).
- Using Internet Recovery Mode (Same as above).
- Looking at Verbose startup mode -- it said "Error 13" on the drive IO.
- Talking to the "geniuses" at an Apple Bar, as well as online tech support. They suggested either formatting my drive, or even buying a new drive. They had no inclination to dig into the problem and actually figure it out and fix it.
What worked:
- I put my Mac in Target Disk mode (hold down T upon startup).
- I connected my Macbook to another Macbook using a Thunderbolt cable. The other Macbook must be running High Sierra. If you don't have a friend with a Macbook, you might be sh*t out of luck.
- I made a backup of my files, in case things went poorly. My Macbook appeared as a drive on my friend's High Sierra computer, but dragging files over in Finder resulted in an error. I had to use Terminal and "cp" my files over.
- I opened Disk Utility on my friend's computer, and here are the steps I took:
- View -> Show All Devices
- My Macbook showed up, along with various "Container disks".
- I ran First Aid/Repair on all of the various things I saw. Some of them threw errors and refused to repair, but that seems to have been okay.
- I then created a bootable USB installer as per How to create a bootable installer for macOS - Apple Support using my friend's computer.
- I ejected my Mac, disconnected it from the other computer, and booted my Mac onto the USB booter. I ran the installer from the USB stick, and it succeeded! After it completed, High Sierra booted up, and all of my files were present, unharmed.
This was a huge headache for me, and I hope that this writeup helps save your files. Don't necessarily listen to the Apple "geniuses" who say that a format is necessary.
