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If you go into disk utilities you can watch it only allow one external drive run PCI-Express…
The rest default to USB.
Drives:
Two Scandisk Pros (1TB&2TB) and one Samsung 990 in a Orico M234C3 enclosure.
Note:
If you don’t plug the Samsung 990 in first, it never finds it at all…
the drive that gets plugged in first gets PCI-Express assigned and read writes at the proper speed, 2k+, the remaining never go above 900.
not good :(
another possible culprit is Cables. genuine Thunderbolt cables carry the Thunderbolt symbol, and have TWICE the number of data pathways as USB cables. they can reach the full 40 G bits/sec expected speeds.
USB-C cables that carry the SuperSpeed PLUS logo shown below can attain UP TO 20 M Bits/sec on ports that reverse the inbound and outbound lanes for maximum speed when needed:
cables with no logos or certification tags and not good for high speed transfers at all.
cables longer than about ONE meter may not be good for high-speed transfers either.
"shipped in the box' cables are often "lowest bidder" cables, and are NOT guaranteed to provide top performance.
Intel ThunderBolt-3 Macs can get up to nominal 2500 M Bytes/sec on an appropriately fast SSD drive in a ThunderBolt-3 enclosure, provided they use a non-busy ThunderBolt controller on the Mac (not shared controller busy servicing displays or other drives on a second Mac port) supported by a typical 32 G bits/sec (PCIe2 x4, or PCIe3 x2 amount of bandwidth). Apple-silicon is similar except each FRONT Thunderbolt port is supposed by its own ThunderBolt controller.
If you get 2,000 M Bytes/sec out of one drive, you have already taken 2/3 of the available performance of that bus. You need to put some of those drives on another Bus.
Possible that the the TB bus cannot handle the load. I have some TB external drives that will not even be recognized if I have more than one attached at the same time.
I need to clarify what I wrote above. FRONT USB-C ports on Mac Studio with MAX processor (not Ultra). Those front ports are USB, and limited to 10 G bits/sec maximum (by specification).
I'm also having the same issue on a MacBook Air 15, the first drive connects via PCI-Express, and the second drive is using USB.
Why will Only one external Thunderbolt drive run at full speed?
