2015 21" iMac 4K [5th,2C,U] + RX Vega 56 Nano @ 16Gbps-TB2>TB3 (Gigabyte Gaming ...
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2015 21" iMac 4K [5th,2C,U] + RX Vega 56 Nano @ 16Gbps-TB2>TB3 (Gigabyte Gaming Box) + macOS 10.15.2 & Win10 1903 [itsage]  


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Joined: 4 years ago

This late 2015 iMac 21.5" came in to be wiped and recycled. There wasn't anything wrong with it so I made it into a Thunderbolt 2 eGPU testing system. It's as base as base can get but the performance is surprisingly decent.  😀 


System specs:

2015 21" iMac - i5-5250U/HD Graphics 6000 iGPU/8GB RAM/1TB HDD


eGPU hardware:

Gigabyte Gaming Box + RX Vega 56 Nano + Apple Thunderbolt 3/USB-C to Thunderbolt adapter & .5m Thunderbolt cable


Hardware pictures:


Installation steps:

Thanks to @Mac_editor's Purge-Wrangler script, the late 2015 iMac can use AMD eGPU in macOS Catalina. The RX Vega Nano Gaming Box worked through hot-plug and cold-plug as if it's natively supported. I tested external monitor output through an LG 4K and that worked well too.

It is a lot more work in Windows Boot Camp however. I was hoping with only the iGPU, it would be more straight-forward. Unfortunately error 12 was the culprit. Interestingly an Nvidia eGPU can work through hot-plug in Windows desktop (OS Build 18362.30). In order to solve error 12, I followed @nando4's instruction to do DSDT Override. Once I had the modified DSDT file, I used it with @goalque's automate-eGPU EFI boot manager to allocate Large Memory in Win10. Here are the step-by-step:

  • Install Windows 10 ISO through Bootcamp Assistant [in macOS]
  • Install DDU to remove all default graphics drivers and disable Windows automated driver installation [in Windows]
  • Create a 24MB FAT partition in Disk Utility and copy @goalque's automate-eGPU EFI onto it [in macOS]
  • Copy the 2015 21" iMac dsdt-modified.aml file to /EFI/CLOVER/ACPI/WINDOWS/dsdt.aml inside the EFI partition [in macOS]
  • Hold OPTION at boot to select EFI drive and hit Q to access automate-eGPU EFI [boot selection]
  • Install Intel iGPU drivers for the integrated graphics card then restart [in Windows]
  • Shut iMac down then connect eGPU to Thunderbolt port closest to Ethernet port [system OFF]
  • Boot into Windows via automate-eGPU EFI [boot selection]
  • Confirm a new Microsoft Basic Display Adapter in Device Manager then install Radeon drivers [in Windows]



In macOS I ran CL!ng and Luxmark 3.1 to check bandwidth and compute performance.

I ran Unigine and 3DMark synthetic benchmarks in Windows Boot Camp. First was internal display mode then next was external monitor mode through the LG 4K FreeSync monitor.

Internal Display External Monitor



Apple Boot Camp drivers for the Intel iGPU were very old and produced bad performance. DDU not only helps removing them completely but also prevents Windows from installing older drivers by itself. Best performance is through an external monitor but the appeal of the iMac is to drive the internal display. An iGPU-only Mac allows AMD XConnect to accelerate the internal display automatically. Thunderbolt 2 reduced overall eGPU performance but it's significantly better than the iGPU.

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