I've also tried booting into Safe Mode and the progress bar gets almost to the end before rebooting. I've reset PRAM/SMC multiple times to no avail. I've tried a fresh install without the display connected but the installer keeps rebooting. I have just done a fresh install of Lion and then updated to Mavericks and then High Sierra which it's running now. It will stay in this reboot cycle until I power off and connect the external display. If I disconnect the external display and try to boot I see the Apple logo with the progress bar underneath which gets about half way along and then the machine reboots. I have a mid 2011 27' iMac that has a strange problem: it will only boot successfully if an external display is connected via a Thunderbolt port. IMac won't function without an external display connected If I tried to boot without a display connected it would simply get stuck in a loop.Īdmittedly it's been a while since I've attempted to resurrect the Mac again but does anyone have any suggestions for what I could try or what might be the problem? The main goal is just to re-install a working version of MacOS again. But since then the Mac will only boot if one of the external displays is connected. This caused the Mac to crash and get stuck in a reboot loop which I finally got around (can't remember exactly how). Not sure if that gives any clues as to a possible hardware problem or whether it's simply that Snow Leopard won't run on a 2011 iMac.Ī bit of further background: I used to have two external displays connected to the iMac and during a recent MacOS update I foolishly decided to disconnect one of the displays. I even have original install DVD for Snow Leopard (10.6) which I've tried but I get the following linux screen: I've tried installing both Majove (10.13) and High Sierra (10.12) from a USB drive and the problem is either that it gets stuck in a reboot loop, or I see the non-bootable disk icon when trying to boot from the USB drive. I've got a 2011 iMac that I can't seem to re-install any version of MacOS on.

External graphics card for imac 2011