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iMac mid 2011 21.5 Graphics issue

  danpoulter 07:02 07 Jun 2018

Hi all, I am having some issues with an iMac which I have recently changed the HDD in. The Mac boots fine until it gets to the login screen, then odd horizontal lines start appearing. This will then cause the Mac to freeze (around 2-5 minutes) at which point I have to cut the power to reboot it. The Mac boots fine in safe mode and will work indefinitely. I have rebuilt the Mac on High Sierra, Sierra and Yosemite but on all occasions I get the same issue when booting. Interestingly I booted the iMac without the screen connected and with an external monitor instead, this worked fine even though it was booted normally and not into safe boot. If any one has any advice it would be greatly useful.

BTW the Mac is an i5 2.5GHz with 8GB RAM and an AMD Radeon HD 6750M 512MB.

Thanks in advance.

  HondaMan 16:42 07 Jun 2018

Sounds like it could be memory fault. Have you tried "memtest" or some other memory checker. The fact that all is well in safe mode could point to a device driver, probably graphics as the external monitor is OK. Have you tried connecting the iMac monitor to another computer to make sure it's not the monitor itself

  danpoulter 18:46 07 Jun 2018


Thanks for the reply. I have tried using different SODIMMs of RAM in the iMac but it always appears to have the same issue. The PC is an iMac so the monitor is not interchangeable as it is 'built in' as such. The external monitor is only okay when the actual iMac screen is not connected.

I will try memtest and see how I get on.


  bremner 12:13 08 Jun 2018

Odd horizontal lines and that it is fine with an external monitor shout defective graphics card.

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