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User profile for user: robomartion
robomartion Author
User level: Level 1
11 points

MacBook Pro usb-c speed switching from 3.1 to 3.0 bus causing playback issues in Final Cut Pro X

I am now aware that there are two buses in the MacBook Pro 2017 13" one for usb 3.0 and one for usb 3.1. At first I thought one port is USB 3.0 and one is 3.1 but they seem to switch on the fly.


Because of this I am unable to use an external drive for Final Cut Pro X because the speed keeps switching to USB 3.0 speeds which is only 40MB/S rather than USB 3.1 speeds which are 500MB/S which makes playback choppy. As soon as I notice the slowdowns I can perform a disk speed test which corroborates that the bus has been switched.


Is there a setting to prevent this switching?


MacBook Pro

Posted on Jul 22, 2021 8:08 PM

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User profile for user: robomartion
robomartion Author
User level: Level 1
11 points

Jul 23, 2021 6:40 PM in response to Luis Sequeira1

The device is a Samsung 860 QVO 2TB connected via a SATA 3 to USB-C connector. In these two screenshots you can see the speed difference 480 Mb/s (megabits) vs 5 Gb/s (gigbits) which changes to the slower speed randomly, then if I switch the port it is connected to the speed is fixed again then randomly it might switch to the slower speed again but sometimes it stays full speed for a while.


Another identical 2TB SSD has the same issue (but with which it seems to happen less) with the exact same connector but the exact same USB-C connector does not have the same problem in an older mac using a standard USB-A 3.0 adapter.


To me it sounds like the PCIe internals of the MacBook pro have a limited amount of bandwidth and it is allocating them randomly, otherwise the connector or SSDs are at fault but I don't see why if it's working fine on another MacBook.


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User profile for user: robomartion
robomartion Author
User level: Level 1
11 points

Jul 23, 2021 10:37 PM in response to terryb

The other connector I have is a caddy with a SATA to USB 3 header to separate USB-C cable which doesn't have the same problem. The only thing is the original connector the topic was about seems to work fine currently too, or was for about 30 minutes until the battery went to about a quarter and the fans ramped up then the connector went back to USB 2.0 speeds.


So either the connector has some problem or just isn't fully compatible with the way the MacBook pro usb-c ports work. I have a feeling it could also be to do with power draw or the computer thinking it is a charge cable but I am not entirely sure. Some similar sata to usb-c connectors include ac power so that could potentially be what it is.


For interests sake the direct SATA to usb-c connector reaches 60MB/s and 100MB/s faster than the SATA to usb 3 caddy with the same SSD.





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User profile for user: Luis Sequeira1
Luis Sequeira1
User level: Level 9
67,582 points

Jul 23, 2021 12:59 AM in response to robomartion

There is a bit of confusion here. The speed you are talking about suggests USB 2, not 3.0.


You are probably having this if you are adding a USB2 device to the same bus.


You need to tell us specifically what mac, what drives, what adapters or docks you are using and what other devices are connected, and how.


EDIT: sorry, you did say what mac it is… but do tell us the rest

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MacBook Pro usb-c speed switching from 3.1 to 3.0 bus causing playback issues in Final Cut Pro X

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