Late 2015 iMac. Fastest Data Transfer from new USB-C devices

Hello, Thank you if you can help...


After much researching, I still cannot establish which is the fastest port on my iMAC AND from which there is an adaptor to accept new USB-C devices.


For some reason the vast majority of adapters are the wrong way round, ie, they will connect multiple connector types, female, to USB-C, male, but not vice-versa. ... as if they cater for connecting old devices to new computers, and not old computers to new devices!


... any suggestions please...?


TIA.




iMac 27″, macOS 10.13

Posted on Sep 7, 2023 4:09 AM

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Question marked as Best answer

Use USB-C to USB-A adapters in the iMac's USB 3 ports.



have a look at > usb-c to usb-a adapters

Posted on Sep 7, 2023 5:18 AM

22 replies

Sep 8, 2023 9:21 AM in response to Martin-123

Martin-123 wrote:

This is very disappointing! ... but thank you, ku4hx.
... so on the Late 2015 model do you just live with 559Mb/s or have you found a work-around?

Thunderbolt 2, "a dead issue" ...? Can you say a bit more about it please...?

I have a late 2015 iMac also. The USB 3 ports are generally plenty fast for what that computer is capable of doing.


I was interested in making use of the Thunderbolt 2 ports so I obtained a Thunderbolt 2 to 4 adaptor and also an inexpensive powered Thunderbolt 4 hub from OWC. They were on special for about $100, it offers multiple USB-C compatible and Thunderbolt 4 ports and even another USB 3 port. So with the Thunderbolt 2 to 4 adaptor in combination with the powered Thunderbolt 4 hub, one can connect multiple USB-C or Thunderbolt 3 (or 4) devices to the iMac through the powered hub. It has to be a powered hub because cause that adaptor doesn't carry power, so the hub needs its own power. This is not an issue for a desktop computer like an iMac, but might be inconvenient for a laptop in such a setup.


The speed of all the devices will be Thunderbolt 2 speed (not Thunderbolt 4) but that is is still 20 Gb/s, faster than the iMac can keep up with.


With this setup, I have the original 4 USB 3 (5 Gb/s) ports in the back of the iMac, plus three Thunderbolt 4 (or USB-C) ports in the hub (20 Gb/s at Thunderbolt 2 speeds), plus another USB 3 port in the hub. There remains an unused Thunderbolt 2 port on the iMac but since Thunderbolt 2 devices aren't available anymore, that just doesn't get used. But this expands the number of connections available way beyond anything I can actually use.

Sep 8, 2023 9:38 AM in response to Martin-123

P.S. OWC MacSales still offers the inexpensive Thunderbolt 4 hub (powered, which is what you need) and also sells the Apple Thunderbolt 2 to 4 adaptor that would enable you to connect a Thunderbolt 4 hub to one of your Thunderbolt 2 ports on the back of your iMac, like I did with mine. That gets you 20 Gb/s for any USB-C device connected to that hub.

Sep 8, 2023 10:15 AM in response to Martin-123

Martin-123 wrote:

It sounds like exactly what I'm looking for, Steve. Thank you again, but what about this...

"... but that is is still 20 Gb/s, faster than the iMac can keep up with."

... confuses me a bit. Sorry for my lack of knowledge. Could you say what you mean by that please? Is it just a joke on Apple...?

Depending on your RAM, an older Mac like that and its peripherals (external drives, e.g.)( can only process data and information so fast. Probably the most intensive applications would be high speed video editing and processing, and what I meant was that the throughput of an older Mac like ours in real life intensive applications would top out before reaching the 20 Gb/s limitation of the external connection. So using this setup means your flow is limited primarily by the processing ability of the computer and its peripherals, not by the speed of the external connections to the large data files.


A simple example would be doing a backup to an external SSD through this 20 Gb/s (which is like ~ 2.5 GB/s) connection. Let's say your external SSD is capable of 1 GB/s, which is a very fast SSD. You will never be limited by your 20 Gb/s (2.5 GB/s) connection because the external SSD can't go faster than 1 GB/s. When I do Time Machine backups or SuperDuper backups to such external SSDs, the fastest I have ever seen in practical use is about 1 GB/s, even though the device is actually rated faster. So I have never bumped up against that 20 Gb/s (2.5 GB/s) limit of the Thunderbolt 2 port. The Time Machine or SuperDuper application has to do many calculations and file manipulations during the backup, that slows down the throughput to below the fastest that the Thunderbolt 2 can support.


I think instead of saying "that is is still 20 Gb/s, faster than the iMac can keep up with" I should have said "that is is still 20 Gb/s, faster than the iMac+external_peripherals can keep up with"


By the way, you would be fine without the Thunderbolt hub and its additional cost, using instead the built in USB 3 ports. USB 3 on that Mac gets 5 Gb/s, which is about 600 MB/s, and that's really fast for almost anything. The reason I got the Thunderbolt 4 hub and adaptor was to prove to myself that I could maximally exploit all the external ports on this old iMac, even the Thunderbolt 2 ports (which are obsolete but can still be utilized with said adaptor).

Late 2015 iMac. Fastest Data Transfer from new USB-C devices

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