My iMac won't boot past the grey screen with progress bar. I am running High Sierra. I've tried starting in safe mode with the same results. In recovery mode the disc first aid says the disc is OK. I've also reloaded the operating system.

I have the solution but you are not going to like it! My iMac started this when I installed Boot Camp and Windows 10 Pro. All was well until I rebooted from Windows back to the High Sierra 13.4 partition. I got a GRAY screen. Could not get around it. Tried Windows and it loaded just fine.

Multiple OS reinstalls, every disk check possible. Turned out that the Formatting on the SSD was trashed. The only solution was to REFORMAT the SSD to its original configuration. Mine ended up at Sierra 12.6.

After launching successfully had to reinstall all software from originals or the App Store. The Time Machine backup got trashed when it backed up the messed up SSD. Lost a ton of work. Now I put crucial work on DVDS along with Time Machine and an external drive that I just Drag & Drop on. Updated to High Sierra last night and all is well.

Good luck!

Then you likely have a hardware issue such as a crash HD. The HD appears OK is not a thorough test, you will need to take it to your local Apple Store or Apple Authorized Service Provider to be thoroughly tested.

I finally had to take it to a computer tech and he had to wipe the hard drive and reload the software as though a new machine. I didn't have everything backed up and lost everything.

Same thing happening to me. I've tried safe mode and recovery mode, tried disk utility and time machine restart. Tried reinstalling os high sierra but message says "not authorized". Nothing is working.

Me too Allen, did you find a solution?

Sorry that you had to go through that! At least your Mac is back!