Chosen Solution
Hi to everyone, I have an Imac 21.5 emc 2428 stuck on loading bar. I’ve opened it ed it was full of dust and change thermal paste(never changed before). The problem remains. Now I can’t open mac recover os. I think this happened after an upgrade to Sierra. I tried a new hd and a new ssd but command + r doesn’t work, alt doesn’t work to choose hd or usb pendrive. The combination that works is alt command and r to activate internet recovery. After I’ve done internet recovery (after logo Apple with loading bar) complete I had an infinite white screen and the fans at very high speed. The strange thing is that the Imac with an hd with installed Win 10 works fine. Thanks in advance.
Your iMac has two graphics services the one within the CPU and a second with the AMD Radeon MXM board. Windows uses the Intel Iris graphics engine, while your macOS uses both! So when the MXM board fails it leaves you at a white (gray) screen. Finding a replacement board will be difficult as you need one with the Apple firmware! iFixit has a few now iMac Intel 21.5" EMC 2428 Radeon HD 6750M Graphics Card and here’s the guide to put it in iMac Intel 21.5" EMC 2428 GPU Card Replacement
