I am seeking suggestions on what may be going in with an iMac I am currently troubleshooting, as it’s displaying behaviour I do not recall seeing before.
This iMac refuses to boot up. It gets the Apple logo, gets past the progress bar, and then gets stuck on the grey screen for about two minutes before rebooting itself. However, what makes this odd is that I can:
- Start the iMac in Target Disk Mode and use it to successfully boot up other Macs. In doing so everything seems to work perfectly fine.
- Successfully boot the iMac from an external drive and everything seems to work perfectly fine.
I was presented this iMac about 2 weeks ago as it refused to bootup. That is, it would commence booting up and cycle through the Apple logo and progress bar, but after the progress bar completed it would remain on the grey screen for a couple of minutes before rebooting itself and doing the same thing all over again.
To address this I undertook the following troubleshooting steps:
- Resetting SMC - failed
- Resetting NVRAM - failed
- Starting up in Safe Mode - failed
- Starting up in OS X Recovery Mode – failed
- Starting up in OS X Recovery Mode via the internet – failed
- Undertook an extensive hardware test using Apple Hardware Test (AHT 3A213) - passed
- Started iMac in Target Disk Mode and retested hard drive using Scannerz on another iMac - passed
- Started iMac in Target Disk Mode and reformatted the hard drive and did a clean install of Mac OS X El Capitan 10.11.6 - successful
- Migrated data from a Time Machine backup - successful
After these steps I tested the iMac for four days with no issues in terms of booting up and using the computer. On the fifth day I needed to manually import seven Apple Mail mailboxes. I did these one by one and six of them imported within minutes.
The last mailbox was showing as being 270 GB in size. I knew this was inaccurate, but I decided to import the mailbox so I could do a mailbox rebuild to correct whatever corruption had taken place. This import took over 12hrs to do but seemed to work fine and Mail was displaying the emails.
I then tested the iMac on and off for another day to ensure it had no problems booting up. This was successful.
I then decided to initiate the mailbox rebuild, but as soon as I did the screen went black and stayed that way regardless of any keyboard shortcuts I tried using. I waited an hour, forced a reboot, and ever since the iMac has returned to its original behaviour of not booting up – it would get past the progress bar and stay on the grey screen for a while before rebooting itself and starting the whole vicious cycle again.
I have again retested the hard drive, processor, RAM and Video Ram by starting the iMac from an external hard drive and using a combination of the following packages:
- Apple Hardware Test (AHT 3A213)
- TechTool Pro
- Memtester (under the Homebrew subsystem)
And now I have a situation where I can boot the iMac from external hard drives, and also use it in target disk mode to boot up other Macs, but cannot use it to boot from its own internal drive.
At present I have isolated the internal hard drive and am running a scan to check for any malware.
In the meantime, I am wondering:
- Has anyone come across a similar scenario and, if so, what was the cause?
- Assuming I find no malware, do I go through the reformat/clean install/migration process again, but this time not import the last mailbox and see whether the iMac works fine?
- Any other suggestions?
iMac specs are as follows:
- iMac (27”, mid 2011)
- 3.1 GHz Intel Core i5
- 8 GB RAM
- AMD Radeon HD 6970M 1024 MB
- 1 TB 3.5” SATA HD
- Mac OS X El Capitan 10.11.6