Why Apple? Why???!!!

July 2, 2020 2:36 PM

My wife has a 2017 iMac running Catalina. Two weeks ago the hard drive died. The hard drive has since been replaced and things seemed to be working until she tried to install Windows via Bootcamp (which she needs for work) and now....nothing works? Help?

So, my wife had trouble with the Windows install not working properly and she had to do it a couple times. Eventually she got Windows working. But then neither Catalina or Windows would boot consistently. Booting Catalina specifically goes like this: Boot starts normally, gets about half-way and then the screen goes blank, and then nothing.

Things I've tried:
1. Booting into recovery mode and running first-aid on everything I could possibly run first-aid on. No problems as far as I can tell.
2. Booting into diagnostic mode. No problems reported.
3. Booting into single-user mode. Boots to command-line fine. Tried to run fsck but it complained (don't remember the error specifically but it wasn't a problem with the filesystem it was a problem with the volume being mounted with write permissions....or something?)
4. Booting into safe mode. Works fine consistently. I get boot into safe mode every time no worries. But restarting to boot normally results in the same problem (get's half-way then apparently stalls.)
5. Deleted the Windows partition and re-installed Catalina (without deleting user data.) No dice, same problem.
6. Clean re-install of Catalina (deleting user data and re-installing from usb install image.) Nope. Had to boot into safe mode to finish the install and set up the user account.

However, I noticed when installing from USB that the boot device selector screen (holding option after turning the computer on) still shows a Windows boot device. That seems odd. I booted into it just to see what would happen and it takes me to a blue screen telling me something is wrong (no kidding!) and when I hit F1 it just reboots the computer and I'm back where I started.

So I have two questions for people who know Macs better than me (basically everyone):

1. How do I make the boot selector stop thinking there's a Windows partition when there isn't one.
2. Since I can boot consistently into safe-mode I assume that the normal boot process is trying to run something which is causing my problem. I assume this has something to do with the failed Windows/BootCamp installs my wife was having trouble with. How do I find the offending software and tell the kernel not to load it?

I have a general understanding of operating systems and boot processes but I am by no means an expert. I'm happy to hack around on a command-line if that'll help.

Thanks in advance, hive-mind!

posted by Mister_Sleight_of_Hand to Computers & Internet (7 answers total)

Best answer: See if this article helps with the Windows partition.
posted by hanov3r at 2:42 PM on July 2, 2020

You might also want to try resetting the PRAM and SMC, that seems to fix a lot of weird things on Macs.
posted by xedrik at 4:37 PM on July 2, 2020 [2 favorites]

Since you seem to be comfortable booting random stuff, and apparently have nothing precious on the disk, you could try booting a Linux install, going to rescue mode, and using dd to really wipe the partition table on your disk. (I'd guess there's some nastiness there, and Windows and Mac installs aren't clearing out everything necessary.)

If you can get to a Linux command line, figure out what your hard disk is (probably /dev/sda):

ls /dev/sd*

Then nuke the beginning of the disk (where the partition tables live):

dd if=/dev/zero of=/dev/sdX bs=1024 count=10

(where X is the disk you found from the ls command)

This should get you to a state where the disk looks totally brand new, and your OS installs should set it up properly.
posted by spacewrench at 5:24 PM on July 2, 2020 [2 favorites]

Amending Spacewrench's offering because the newer guid-driven partition table is 40 512-byte blocks (equivalent to 20 1024-byte blocks):
dd if=/dev/zero of=/dev/sdX bs=512 count=40
posted by k3ninho at 1:01 AM on July 3, 2020 [1 favorite]

Best answer: This iMac might have a bad GPU. Safe mode and all the other modes you e booted to that "work" don't load full video drivers which might be why they continue to work. Sounds like the video output is dying as soon as the GPU drivers loads halfway through the boot sequence - you should setup SSH or something and see if the unit becomes visible on the network despite the lack of video.
posted by denaps at 5:59 AM on July 3, 2020 [1 favorite]

Response by poster: Well crap.

Yep that seems to be the problem, denaps. Thanks for helping us figure it out even though it's not good news.
posted by Mister_Sleight_of_Hand at 6:38 AM on July 3, 2020

Response by poster: Also, thanks hanov3r, that did get rid of our phantom Windows partition which once the computer gets fixed (again) would have still been a problem.
posted by Mister_Sleight_of_Hand at 8:55 AM on July 3, 2020