Q: Solid grey window where I should see a login window

I have opened up the iMac

1) disengaged the rotational drive from SATA and power

2) Rebooted the mac with only SSD attached

3) Tried internet recovery via ethernet. Downloads fine but see no 'choose language' page only a grey screen

4) Tried recovery partition, apple logo and progress bar appear and then a grey screen

5) Tried normal boot, see apple logo and progress bar and then a grey screen

Internal SSD is attached to MBA (MacBook Air) now via SATA to USB connection, mounts fine.

Will boot from the SSD mentioned above to see if MBA boots with it.*

Noteworthy, when detaching the screen, I noticed that the rotational drive's heat sensor had detached itself. The machine was getting hot just before it kernel panicked with a solid grey screen with black vertical lines.

*I will report back to let you know if I can boot from the iMac's SSD using the MBA.

Note: I am working with anti static precautions, wrist strap grounded and grounded matte.

Below is the / directory of the iMac SSD mounted on the MBA , just an FYI.

Bee

The kernel panic log under /Library/Log/DiagnosticReports is empty. I did wait about 5-10 seconds before hitting the power button. I should have waited longer. When it restart though, it booted to a kernel panic screen. This is unfortunate, it could have helped understand the source of the panic but as I've said, the machine was hot before the panic occurred, now I know why, rotational drive's sensor was not attached to mechanical drive.

As promised, here is additional info.

The SSD (named Boot_Volume) starts up MBA

- when booting it actually continued the internet recovery, as begun by the iMac.

The rotational drive (named Macintosh) mounts on the MBA without an issue.

Both passed DiskUtility.

quick update.

Both of the drives are fine, all data is intact.

As you can see in the pic, the MBA is connected to one of the drives, in this case the internal rotational drive.

-I disengaged both drives including the optical drive from the iMac MLB.

Next steps

With all SATA connections vacant, I will startup the iMac from an external USB drive and then to connect using screen sharing. We'll see what happens. I will report back.

NOTE:

I believe I am looking at a hardware repair at this point but I hope this serves as an educational experience for others.

ok, drama is over.

The iMac chimes up and then restarts on its own. Something is wrong with the HW.

Off to the AASP she goes. Hopefully this will not cost an arm and a leg.

Many thanks for chiming in.

Will review OS X: When your computer spontaneously restarts or displays "Your computer restarted because of a problem." - Apple Supp… before anything.

LOGS

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Sun May 29 23:31:34 2016

Event:               GPU Reset

Date/Time:           Sun May 29 23:31:34 2016

Application:         WindowServer

Path:               

OS Version:          Mac OS X Version 10.11.5 (Build 15F34)

Graphics Hardware:   AMD Radeon HD 6970M

Signature:           0

Report Data:

GPURestartReportStart

------------------------

Hung Channels: PM4

------------------------

[00] AccelChannel: PM4

Pending Command from   : GLCtx

PendingCommandTimestamp: 0x0000001c, TotalDWords: 0x0000050b, GART Offset=0x0000000080603400, stamp_idx=0, estamp=0x0000001c

PendingCommandStart:

PendingCommandEnd

------------------------

[00] PM4 HWChannel  : Enabled, NotIdle

LastReadTimestamp   : 0x0000001b

NextSubmitTimestamp : 0x0000001e

[PM4, ts:0x0000001c]: No semaphore wait

[PM4, ts:0x0000001c]: semaphore  signal: 0xffffff803cdf1a00

HWSemaphore 0xffffff803cdf1a00 Signal Event: [PM4 channel[0] TS:0x0000001c lastReadTS:0x0000001b]

------------------------

GPU HangFlags 0x00000004: AsicHangState 0x00000004, AsicResetRequirement 0x00000002

IndividualEngineHang: 0

NonEngineBlockHang  : 0

FenceNotRetired     : 1

PerEngineReset      : 0

FullAsicReset       : 1

HangEngineBitmap    : 0x00000000

------------------------

AMDBartsGraphicsAccelerator PCIe Device: [1:0:0] State: ENABLED

Configuration: deviceBits: 0x00816720, capabilityBits: 0x30940100

TotalVideoRAMBytes: 0x0000000080000000 (2147483648)

------------------------

[00]  PM4 Engine : Enabled,  NotIdle

[00]  PM4 Channel: Enabled,  NotIdle

------------------------

[01] SDMA Engine : Enabled,  Idle

[01]  DMA Channel: Enabled,  Idle

------------------------

[03]  UVD Engine : Disabled, Idle

[02]  UVD Channel: Disabled, Idle

------------------------

[04]  SPU Engine : Disabled, Idle

[03]  SPU Channel: Disabled, Idle

------------------------

: ** GPU Debug Info Start **

: 0x00006720 : 0x00000092 : 0x00000001 : 0x00000018 : 0x0000a880 : 0x00000001 : 0x00000001 : 0x00006720

: 0x00000001 : 0x00000015 : 0x00000000 : 0x00000001 : 0x00000047 : 0x00000000 : 0x00000001 : 0x00000058

: 0x00000012 : 0x00000001 : 0x00000055 : 0x00000000 : 0x00000001 : 0x0000004a : 0x0000106b : 0x00000001

: 0x0000004d : 0x00000b00 : 0x00000000 : 0x00000010 : 0x00e30800 : 0x00000000 : 0x00000021 : 0x00000080

Sun May 29 23:31:44 2016

Event:               GPU Reset

Date/Time:           Sun May 29 23:31:44 2016

Application:         WindowServer

Path:               

OS Version:          Mac OS X Version 10.11.5 (Build 15F34)

Graphics Hardware:   AMD Radeon HD 6970M

Signature:           0

Report Data:

GPURestartReportStart

------------------------

Hung Channels: PM4

------------------------

[00] AccelChannel: PM4

Pending Command from   : GLCtx

PendingCommandTimestamp: 0x00000020, TotalDWords: 0x00000170, GART Offset=0x0000000080b51c40, stamp_idx=0, estamp=0x00000020

PendingCommandStart:

PendingCommandEnd

------------------------

[00] PM4 HWChannel  : Enabled, NotIdle

LastReadTimestamp   : 0x0000001f

NextSubmitTimestamp : 0x00000029

[PM4, ts:0x00000020]: No semaphore wait

[PM4, ts:0x00000020]: semaphore  signal: 0xffffff803cdf1c00

HWSemaphore 0xffffff803cdf1c00 Signal Event: [PM4 channel[0] TS:0x00000020 lastReadTS:0x0000001f]

------------------------

GPU HangFlags 0x00000006: AsicHangState 0x00000006, AsicResetRequirement 0x00000002

IndividualEngineHang: 0

NonEngineBlockHang  : 1

FenceNotRetired     : 1

PerEngineReset      : 0

FullAsicReset       : 1

HangEngineBitmap    : 0x00000000

------------------------

AMDBartsGraphicsAccelerator PCIe Device: [1:0:0] State: ENABLED

Configuration: deviceBits: 0x00816720, capabilityBits: 0x30940100

TotalVideoRAMBytes: 0x0000000080000000 (2147483648)

------------------------

[00]  PM4 Engine : Enabled,  NotIdle

[00]  PM4 Channel: Enabled,  NotIdle

------------------------

[01] SDMA Engine : Enabled,  Idle

[01]  DMA Channel: Enabled,  Idle

------------------------

[03]  UVD Engine : Disabled, Idle

[02]  UVD Channel: Disabled, Idle

------------------------

[04]  SPU Engine : Disabled, Idle

[03]  SPU Channel: Disabled, Idle

------------------------

: ** GPU Debug Info Start **

: 0x00006720 : 0x00000092 : 0x00000001 : 0x00000018 : 0x0000a880 : 0x00000001 : 0x00000001 : 0x00006720

: 0x00000001 : 0x00000015 : 0x00000000 : 0x00000001 : 0x00000047 : 0x00000000 : 0x00000001 : 0x00000058

: 0x00000012 : 0x00000001 : 0x00000055 : 0x00000000 : 0x00000001 : 0x0000004a : 0x0000106b : 0x00000001

: 0x0000004d : 0x00000b00 : 0x00000000 : 0x00000010 : 0x00e30800 : 0x00000000 : 0x00000021 : 0x00000080

: 0x00000000 : 0x00000011 : 0x00000007 : 0x00000002 : 0x00002254 : 0xff84f001 : 0x00000002 : 0x0000263d

: 0x00000001 : 0x00000002 : 0x000021a0 : 0x80878647 : 0x00000002 : 0x0000219f : 0x00068486 : 0x00000002

: 0x0000219d : 0x01000000 : 0x00000002 : 0x0000219e : 0x000d1002 : 0x00000002 : 0x000021da : 0x0a000002

: 0x00000002 : 0x000021db : 0x00000080 : 0x00000002 : 0x000021dd : 0x00000002 : 0x00000002 : 0x000021e0

: 0x00500052 : 0x00000002 : 0x000021e0 : 0x00500052 : 0x00000002 : 0x000021e1 : 0x00f800f8 : 0x00000002

: 0x000021e2 : 0x01fe01fe : 0x00000002 : 0x000021e3 : 0x01b801b8 : 0x00000002 : 0x000021e5 : 0x00b800ba

: 0x00000002 : 0x0000217f : 0x00000000 : 0x00000002 : 0x000021b6 : 0x00000000 : 0x00000002 : 0x00003041

: 0x00000a0d : 0x00000002 : 0x00003040 : 0x00800010 : 0x00000002 : 0x00003042 : 0x00000780 : 0x00000002

: 0x00003043 : 0x80000040 : 0x00000002 : 0x000021c0 : 0x000009e0 : 0x00000002 : 0x00003045 : 0x00000b00

: 0x00000002 : 0x00001508 : 0x00000000 : 0x00000002 : 0x00000b00 : 0x8f200021 : 0x00000002 : 0x00000bd1

: 0x00000000 : 0x00000002 : 0x00000197 : 0x00000010 : 0x00000002 : 0x00000504 : 0x00492491 : 0x00000002

: 0x0000050c : 0x00000002 : 0x00000002 : 0x00000536 : 0x00000000 : 0x00000002 : 0x00000503 : 0x00000000

: 0x00000002 : 0x00002000 : 0x00000010 : 0x00000002 : 0x00002004 : 0xa2733ca0 : 0x00000002 : 0x00002005

iMac (27-inch): AMD Radeon 6970M Video Card Replacement Program - Apple Support

Hmm…

CRITICAL NOTE for any one reading (glad you got this far)
- DO NOT open your machine like I did. I was Apple hardware certified so I am comfortable with what I am doing.

Even so, under AppleCare I wouldn't dream of opening the machine myself.

In this case, I discovered the repair program after the hardware isolation steps and reproducing the issue in order to create log files.

- All info here is for educational purposes only it might help others see an example of troubleshooting.

Oh it is a GPU issue for sure and yes I agree with your note on the AHT (Apple Hardware Test) when reporting NTF (No Trouble Found) as equating 100% reliability, the test is not completely conclusive.

It is the log file and machine's behavior make that are the most indicative

Short recap

1) With no internal SATA devices connected internally and only one external USB OSX boot drive connected, the mac boots up to the progress bar and apple logo, eventually displaying an all white screen

2) Both internal HDs tested and pass Disk Utility using a secondary El Capitan Machine.

3) Single User Mode works

4) Target Disk Mode works

5) Unable to get into a log in window, screen goes white, machine reboots perpetually when left on, log identifies a GPU reset corresponding to reboots.

6) All attempts to start from any media fail if the GPU kext files load (my assumption)

Found Support Article support.apple.com/en-us/HT203787,  iMac (27-inch): AMD Radeon 6970M Video Card Replacement Program

Going to an AASP in a few hours.

I don't agree with you here.

It turns out it was the GPU and Apple replaced it.

Chroot

It was indeed a GPU issue. Apple replaced it.

Thanks!

My system works perfectly after the GPU hardware was replaced. Having my user account on a different drive is not an issue whatsoever. Works splendidly.