Different speeds (factor 10) between left and right ports in MacBook Pro (14-inch, 2021)

I have measured the speed of my external SSD using blackmagic and realised:

The both ports on the left side of the MacBook are 10 times faster in reading and writing then the right side port (37MB/s read and write versus 398 MB/s).

Connection Speed in the System Info is “Up to 480 Mb/s” on the right port, and “Speed: Up to 5 Gb/s” on the left ports.

Can somebody confirm this discrepancy? Or is something amiss with the right port?


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According to this they should all be the same speed.

So it is a problem with mt MacBook Pro?

What kind of SSD are you using, regarding the connection?
Acc. to the information of the link above, all three ports should be able for Thunderbolt4/USB4 which both could get up to 40Gb/s.
But depending on the SSD you are using, or the cable, it could be significantly slower.
Also it might be possible, that the cable “fits” on the left side better, than on the right side. Is there any kind of “play” if you plug in the cable on the right side?
There are also some reports on the internet, that come up with the conclusion of a general problem of a M1 equipped system, and a lack of speed at the Thunderbolt 4 ports. An example is linked below

Same SSD and cable - just plugged in left or right.

Can’t feel any difference in play between left and right. But I tried it again this morning (after switching between left and right several times yesterday), and it is the same speed (fast) now on both. Problem soved for now, but very strange.

Thanks, and I will take an eye on it.


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If memory serves, a couple of years ago there was some brouhaha about if it was better to charge the Mac through the left o right port due to heat & throttling issues. Perhaps you experience reduced performance due to some of the Thunderbolt bridges being warmer (maybe the culprit here is the video card, the CPU, the charging circuitry…) ???

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It seems really that the right one is faulty, i.e. wiggling disconnects the drive. Well - I have to look into this.