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Thunderbolt 3 - How many ports can run at any given time

I apologise if this question has already been asked. I'm sure it must have been but I was unable to find it.


I have the 2017 iMac (Retina 5K) which has two ports on the rear of the Mac which lay beneath the Thunderbolt 3 graphic. To me, that would imply that I can use both ports as Thunderbolt 3 ports. As there are two of these, am I able to use both at the the same time with two Thunderbolt devices (emphasis on running at the same time) or am I limited to just one Thunderbolt 3 device running at any given time? I've received conflicting advice and I'm frankly bamboozled by what I have heard.


Thank you

Posted on Sep 14, 2020 4:39 AM

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Posted on Sep 14, 2020 6:35 AM

I have basically the same iMac as you. The total bandwidth of the two ports is 40 Gbps and it is shared between the 2 ports. Here is a good article that explains the limitations of Thunderbolt 3 and it should answer all your questions and more,


https://macperformanceguide.com/blog/2019/20190128_1352-understanding-Thunderbolt3-bandwidth.html

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Sep 14, 2020 6:35 AM in response to Archaeopteryxx

I have basically the same iMac as you. The total bandwidth of the two ports is 40 Gbps and it is shared between the 2 ports. Here is a good article that explains the limitations of Thunderbolt 3 and it should answer all your questions and more,


https://macperformanceguide.com/blog/2019/20190128_1352-understanding-Thunderbolt3-bandwidth.html

Sep 14, 2020 6:26 AM in response to samtenor

Thank you Samtenor. I think what confuses me is the fact that there are actually two Thunderbolt ports on the single iMac. I had a look at the graph but I couldn't see a straightforward representation of two Thunderbolt 3 drives using both ports at the same time.


I'm trying to make sense of what the local 'Apple Expert' said to me. He seemed to imply that by using one Thunderbolt port for an SSD drive the second would somehow 'weaken' (not the correct word, but I hope you know what I'm trying to say!). Forgive my lack of technical experience and terminology but it sounded like he was saying that one port could be used for the Thunderbolt SSD device so consequently the other SSD could only be used for a slower connection (USB-C)?


Sorry if that sounds confused. I'm trying to piece together what I've been told



Sep 14, 2020 8:15 AM in response to Jeff Donald

Thanks again Jeff. Just a specific question about convenience. With the ports being at the back of the iMac and also the layout of the room, it’s a hassle to plug and unplug devices constantly. If I were to have an SSD in both Thunderbolt ports but strictly only ever use one at a time, do you think I would noticeably affect performance? I know it’s a very specific question but it would help me greatly

Thunderbolt 3 - How many ports can run at any given time

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