2020 27" iMac 5K (RP580X) [9th,8C,K] + RTX 3080 @ 32Gbps-TB3 (Sonnet Breakaway 6...
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2020 27" iMac 5K (RP580X) [9th,8C,K] + RTX 3080 @ 32Gbps-TB3 (Sonnet Breakaway 650) + Win10  


New Member
Joined: 8 months ago

System specs 

iMac 5K i9-9900K, Radeon Pro 580X, 48GB memory, external SSD for games via USB-3 and Win 10 on Bootcamp


eGPU hardware 

Sonnet Breakaway 650W, ASUS TUF RTX3080 OC 10GB, 2 meters 40GBs TB3 cable


Hardware pictures to follow

I am using internal iMac screen.



HW64 Summary

I am new here so I can not post images - so here are all benchmarks and tech data: 

All Benchmarks


Installation steps

1. Install Windows via bootcamp, keep eGPU switched off. Just downloaded latest Windows ISO from Microsoft

2. Install bootcamp drivers automatically

3. no DDU/other tweaks are needed, switched off Windows Update

4. put egpu into sonnet box. Note that this is tough to insert as part of GPU will really touch the Sonnet Box as you try to insert it, 

but it is possible.

5. Windows will detect NVIDIA but would not install proper drivers

6. Install NVidia drivers 497.29

7. then Reboot and you will see in Display Adapter NVIDIA with exclamation and error 43.

8. Then disable 580X and reboot (note that you will have to do it each time you shut down windows)

9. after reboot you will see NVIDIA working and iMac will switch to 4K

10. However Games would be stuck then and barely launching, so you have to re-enable 580X and you will see both 580X and 3080 Active and working, iMac will swith to 5K.

11. Once both 580X and 3080 are active and working you can enjoy gaming. Note that another TB3 port on iMac should not be used

otherwise you will face Code 43 again. so I am using USB-C for my external SSD for gaming.


Benchmarks (Include a  CUDA-Z  or  AIDA64  bandwidth pic. Optional: Valley, 3dmark, noting if it's on internal/external LCD )

All benchmarks on internal monitor as I do not have external monitor.



It can generally handle 4K in modern games at 40-60fps. Going one stop down from 4K will always give you 60fps.

Ray Tracing and DSS generally have 5-10FPS hit depending on the game.

what is weird is that GPU in most cases will only get to 60-70% load max.

I initially thought that this is TB3 limitation, but then on some modern games like Tomb Raider, Cyberpunk it easily gets to 100%.

Another weird thing is that there are occasional freezes during games, trying to work it out as it happens when my SSD load gets to 100% for some reason. 


More updates soon


This topic was modified 8 months ago
2020 27" iMac 5K (RP580X) [9th,8C,K] + RTX 3080 @ 32Gbps-TB3 (Sonnet Breakaway 650) + Win10 [build link]  

itsage, Mini i5 and nando4 liked
Founder Admin
Joined: 6 years ago

@john3bet, Thank you for sharing this build. Looking forward to seeing your experience with loopback eGPU mode on the iMac. Many apps and games do not have good optimization for eGPU mode so often time you don't see full 100% load.


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