iMac w/High Sierra won't boot

With the machine powered off, press the D key then press the power key. You can release the D key when you see the Apple Logo. The machine will come up in the Apple Hardware Test software. Run the extended tests. If the extended tests report no issues, your hardware is (probably) okay. Shutdown and repeat this process, only this time use the R key to boot into Recovery Mode. Go into Disk Utility and run First Aid on your disk(s). When this completes, you can try rebooting normally. If it still fails, once again boot into Recovery Mode and select Install macOS, which will reinstall the operating system. If this fails, it is time to contact Apple Support. Good luck to you.

I know this may not apply to most people, but I know there's got to be at least one person that this may help with this mysterious problem. Let me also add that I've been using Macintosh computers since OS 9 and I'm currently 29 years old, so that's a very large portion of my lifespan; I'm no n00b.

Anyway, this problem was occurring after I tried to switch from Windows 10 BootCamp back into High Sierra. Yes, I did switch to APFS/AFS and it makes total sense that Win 10 hasn't been updated to see the partitions within the file explorer. What was stumping me threw me for a loop! No matter what key-combo I utilized I wasn't getting ANYWHERE; no OS selection screen when holding Option, no Apple Self-Diagnostics holding D, no internal Recovery (Command-R), no Internet Recovery (Option-Command-R with/without Shift), Verbose with V, nor Single User Mode with S, and "fuggetabout" Safe Mode. It would go straight into the Win 10 startup; never have I been so angry at that logo (and believe me I have been before.)

I was about ready to throw in the towel when it dawned on me, I have an already outstanding issue with my keyboard/trackpad regarding the internal non-removable battery, it's SWELLING. Yes, I know.

Don't scold me, I've worked for Apple in the past and I don't have any AppleCare, so walking in there with my tail between my legs is an EXTREME last resort; geeks in my situation should understand. Anyway, due to the swelling, the whole user surface is slightly convex, causing many of the keys to behave erratically, some have even ceased to work at all, mainly the MODIFIER and left-side Command key(s). Normally I use an Apple branded, external, wired, full keyboard as a workaround.

So in an act of desperation, after doing both SMC and NVRAM resets respectfully, I was staring into the black mirror that was my MBP and gently ran my hands over the entire built-in keyboard, making sure to press each key at least twice. This was done to just make sure none of the keys are being "accidentally" pressed or stuck during start-up which would absolutely interrupt every recovery method mentioned. When I felt the keyboard massage was suffice, I retried entering Internet Recovery mode as a starting point taking into consideration that it's possible my SSD file structure is corrupted (however unlikely)...

TA-DA! I was able to see that euphoria inducing minimalistic globe spinning away as the progress bar filled up. Like I said, this may not apply for everyone, but just putting it out there. Hope it helps!

Possible Fixes for El Capitan or Later

You should try each, one at a time, then test to see if the problem is fixed before going on to the next.

Be sure to backup your files before proceeding if possible.

1. Shutdown the computer, wait 30 seconds, restart the computer.
2. Resetting your Mac’s PRAM and NVRAM
3. Reset the System Management Controller (SMC)
4. Start the computer in Safe Mode, then restart normally. This is slower than a standard startup.
5. Repair the disk by booting from the Recovery HD. Immediately after the chime hold down the Command and R keys until the Utility Menu appears. Choose Disk Utility and click on the Continue button. Select the indented (usually, Macintosh HD) volume entry from the side list. Click on the First Aid button in the toolbar. Wait for the Done button to appear. Quit Disk Utility and return to the Utility Menu. Restart the computer from the Apple Menu.
6. Create a New User Account Open Users & Groups preferences. Click on the lock icon and enter your Admin password when prompted. On the left under Current User click on the Add [+] button under Login Options. Setup a new Admin user account. Upon completion log out of your current account then log into the new account. If your problems cease, then consider switching to the new account and transferring your files to it - Transferring files from one User Account to another.
7. Download and install theOS X El Capitan 10.11.6 ComboUpdate or 10.12.6 Combo Update or macOS High Sierra 10.13.2 Combo Update as needed.
8. Reinstall OS X by booting from the Recovery HD using the Command and R keys. When the Utility Menu appears select Reinstall OS X then click on the Continue button.
9. Erase and Install OS X Restart the computer. Immediately after the chime hold down the CommandandRkeys until the Apple logo appears. When the Utility Menu appears:

1. Select Disk Utility from the Utility Menu and click on Continue button.
2. When Disk Utility loads select the drive (out-dented entry) from the Device list.
3. Click on the Erase icon in Disk Utility's toolbar. A panel will drop down.
4. Set the Format type to Mac OS Extended (Journaled.)
5. Click on the Apply button, then wait for the Done button to activate and click on it.
6. Quit Disk Utility and return to the Utility Menu.
7. Select Reinstall OS X and click on the Continue button.

By the way when I got into Recovery Mode I opened up the Terminal and using bless:

bless -mount /Volumes/Macintosh\ HD/ -setBoot

*assuming you're using the default HDD name

to select the macOS partition, although you could also just click the  in the menubar, and select "Startup Disk."

Make an appointment at the Apple Genius Bar for service. If you need to find an Apple Store - Find a Store - Apple. You have two different hardware errors - one for RAM and another for the HDD. Or replace them and see if your problem is solved.

I still stand by my last suggestion to take it in for service to get the memory and/or disk drive fixed or find out if it's some other hardware failure once the run a diagnosis.

If the drive has suffered a motor failure problem then you cannot rely on such repair routines to actually work. BTW, the command line you used starts with "fsck_hps" but should be "fsck_hfs" I believe.

Same model as mine. Only worth around $400-500 dollars now.

Mine had serious problems first week I got it, from memory it was the hard drive and something else. I threw it back at Apple and after previous problems with previous purchases, persisted in getting it pushed up the organisation.

Eventually it was taken over by a manager in Singapore and she arranged a pick up and return and followed up to check everything was OK after. I haven't had a problem since, but I have been very wary of overheating. If it sounds like it is stressing out I put a frozen cloth over the top left edge as an ice pack. We have had some very hot days post Christmas so somedays that is repeated every hour or so. If it gets really hot, I shut the Mac down.

I've lost too many iMacs due to overheating. Apple is really hard to budge on big things.

Peter

Get a different keyboard. Check the BlueTooth pairing, and if the keyboard needs charging or new batteries.