iMac (mid 2011, running OS X 10.12) gets stuck on white screen (after playing the chime sound and displaying the Apple logo and progress bar) when I try to
- boot normally
- boot in safe mode (holding down Shift while booting)
- boot in recovery mode (pressing Cmd+R while booting)
Sometimes the progress bar will run all the way (followed by a white screen), sometimes the white screen interrupts before the progress bar is finished. The progress bar for recovery and safe mode looks the same as the normal progress bar—not sure if that's normal for Sierra but it seems strange, as if it's not really booting into safe mode or recovery mode.
Resetting NVRAM (Cmd+Option+P+R) and SMC (disconnecting power for 30 seconds) didn't help.
Booting while pressing Option gets me to the partition selection. There, I can choose between my normal boot drive (256 GB SSD with the OS), my Windows 8 partition and the recovery partition. Booting to Windows results in a black screen right before the login screen would normally appear. Booting to the recovery partition results in a white screen.
Booting in verbose mode (Cmd+V while booting) rushed through a wall of text seemingly normal before getting stuck on white screen again when—I would assume—the login screen would have shown normally.
I was able to enter Apple Hardware Test once by pressing D during boot, but it appeared to be stuck after a while, so I didn't let it finish. If I'm holding down D now while booting, the machine enters Internet Recovery Mode, same when I hold down Cmd+Option+R. Entering Internet Recovery Mode produced the -6003F and -6003D errors once each. After that it got to the progress bar and the estimated time remaining, but appeared to get stuck (time remaining was 24 hours and not moving, same for the progress bar). When I tried again, it got stock on the "Starting Internet Recovery / This may take a while" screen (unless it might take an hour in which case I might have ended it prematurely).
I was able to start the machine up in Target Disk Mode (T during boot) after connecting my MacBook via Thunderbolt 2. I was to able to completely mirror both, the iMac's SSD (OS disk) and the HDD. No files seem to be corrupted.
Curious detail 1: When trying to boot either normally, into safe mode or into recovery mode and the white screen interrupts the progress bar, in that moment the screen brightness is slightly dimmed.
Curious detail 2: The chime sound was notably quieter for all the boot attempts until I reset the NVRAM. The first of the two chime sounds that play during that procedure was still quiet, the second one was at normal volume again.
Curious detail 3: All built-in LEDs of my Matias Tactile Pro keyboard (1. caps lock, 2. the key that would be num lock on a Windows keyboard and 3. F14) light up for a few seconds immediately after turning on the iMac. After a few seconds they turn off again. When the iMac was operating normally this never happened.
Question: Is it safe to assume that something inside is fried and there'll be no way around a visit and repair at the Apple store?
Edit: I guess one remaining option is trying to boot from a flash drive. Will try that if it turns out that Internet Recovery really is stuck when I check its status again tomorrow.
Edit 2: As per klanomath's suggestion in the comments, I tried booting the iMac from the MacBook's OS disk via TDM and vice versa. When the MacBook is in TDM and I select its OS disk as the disk to boot the iMac from, it does the progress bar into white screen charade on the iMac. When the iMac is in TDM and I try to boot the MacBook from the iMac's OS disk, the MacBook's screen stays dark after the chime sound. It doesn't even get to the disk selection screen while holding Alt. Also, it takes considerably longer (12 to 20 seconds roughly) for the chime sound to play on the MacBook after turning it on when the iMac is connected via Thunderbolt while the iMac is in TDM. Without the two being connected (or when the iMac is turned off), the chime sound plays after maybe 3 to 5 seconds after turning on the MacBook.