Internal NVIDIA graphics card with AMD eGPU

An eGPU requires Thunderbolt 3. Your iMac does not support Thunderbolt 3. That is the only connection fastest enough for the graphics/video data. Here is Apple’s Support document on support and use of an eGPU,

https://support.apple.com/en-us/HT208544

If you need faster graphics than your 2013 iMac, your best bet will be the new 2020 iMac. As an alternative you could look into an Apple Refurbished iMac that has Thunderbolt 3.

It's not that simple.

Thunderbolt 1: 10Gbps + 1x Display Port 1.1 channel

Thunderbolt 2: 20Gbps of combined data + video;

• Max 1x Display Port 1.2 Channel
• Max 16Gbps data (PCIe 2.0x4)

Thunderbolt 3: 40Gbps of combined data + video

• Max 2x Display Port 1.2 Channels per bus
• Max 32 Gbps data (PCIe 3.0x4) per bus
• Min 10 Gbps data guaranteed available per port
• Max 22 Gbps data (32 - 10) per port.

the eGPU could be used to drive an external display and work in conjunction with the 780

software for CUDA can be found here

https://www.nvidia.com/en-us/drivers/cuda/mac-driver-archive/

I'm on a 2013 4GB 780M from work, but I have a 2010 Pro with a stock ATI and a 1660ti running Windows 7 because I use CUDA for 3D work. Apple left Nvidia users in a lurch and support generally comes from Nvidia when they feel like it. The really sad part is so much 3D and AI work is leaning towards Nvidia right now.

There is no official Apple support for any eGPU on anything

other than a Thunderbolt3 Mac.

Any solutions will be 3rd party or from the open source

community (read system hack) that should be done

at your own risk and note that any of the security updates

that will be available for the next two years could break everything.

To that end you can start here:

https://egpu.io

For editing try using OpenGL/Vulcan/Metal instead of CUDA if available.

For CUDA web drivers you have to downgrade back to High Sierra. (Also CUDA is Nvidea specific, so an AMD eGPU would not provide CUDA support).

Apple does not provide native Thunderbolt-1 eGPU support. Open source third party software can enable TB1 eGPUs, but TB1 has only half the data bandwidth of TB3 and will suffer significant bandwidth throttling that gets proportionally worse with faster cards. The 780M is fast enough that I would not recommend trying to improve upon it with a TB1 eGPU.

If you need faster graphics I'd suggest a newer computer.

Are there any settings in Blender Preferences/Settings to enable use of graphics card?

If you right click on the Blender App and click>>Get Info<< do you see and options to enable graphics card/rendering?

What specs are the minimum for Blender to use a graphics card? Does your Mac meet the minimum specs for graphics card?

Speeds for Thunderbolt are as follows:

Thunderbolt 1 = 10 Gbps

Thunderbolt 2 = 20 Gbps

Thunderbolt 3 = 40 Gbps