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User profile for user: DominiekC
DominiekC Author
User level: Level 1
4 points

Macbook Pro M3 Pro : External SSD with Thunderbolt 3 port not available in Finder

I have a SanDisk PRO-G40 external SSD that supports Thunderbolt 3.

On my Mac Studio M1, that SSD connects normally and is also available in finder as a drive. In the "System Information", it is visible in the "Thunderbolt/USB4 Device Tree" as "Device connected" with a speed of up to 40Gb/s. As "Storage", the volume is shown with APFS as file system and PCI-Express as the protocol. The external drive works normally and is available in the Finder.

When I connect that drive to my MacBook Pro M3 Pro using the same cable, I also see it in the "Thunderbolt/USB4 Device Tree" as "Device connected" at up to 40Gb/s but ... it does NOT appear in "Storage" nor does it appear in the Finder. At none of the 3 ports.

Both the Mac Studio and MacBook Pro are running under Samona 14.6.1. On the MacBook Pro, "Allow accessories to connect" is now set to "Always".

The only work-around I've found is to use an Anker 555 USB-C Hub and connect the SanDisk PRO-G40 external SSD through one of the USB-C ports on the Hub, but then the speed is limited to 10 Gb/s.

Why does a direct connection to the MacBook Pro M3 Pro Thunderbolt 4 ports not work but to the MacStudio M1's (rear ports) one does ?

Something wrong with the backward compatibility of the Thunderbolt 4 ports (to Thunderbolt 3) of the MacBook Pro M3 Pro ?

MacBook Pro 14″, macOS 14.6

Posted on Aug 27, 2024 4:29 AM

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14 replies
User profile for user: DominiekC
DominiekC Author
User level: Level 1
4 points

Sep 2, 2024 10:02 AM in response to DominiekC

An update : testing with an original Apple Thunderbolt 4 cable of 1 meter.

  1. The SanDisk PRO-G40 was available on the Mac Studio M1 Max using a Thunderbolt 4 port from the back (Thunderbolt 3 Device connected, Up to 40Gb/s).
  2. Eject the disk properly (right-click menu)
  3. Connect the disk to the MacBook Pro M3 Pro Thunderbolt 4 port : the SanDisk PRO-G40 comes available in the finder.
  4. Eject the disk properly (right-click menu)
  5. Reconnect the disk to the Mac Studio M1 Max using a Thunderbolt 4 port on the back : SanDisk PRO-G40 not available in the finder (but in the "Thunderbolt/USB4 Device Tree" of the System Information as "Device connected" at up to 40Gb/s; not available as "Storage")
  6. Disconnect
  7. Reconnect the disk to the MacBook Pro M3 Pro Thunderbolt 4 port : SanDisk PRO-G40 not available in the finder (but in the "Thunderbolt/USB4 Device Tree" of the System Information as "Device connected" at up to 40Gb/s; not available as "Storage")
  8. Disconnect
  9. Reconnect the disk to the Mac Studio M1 Max using a USB-C port on the front : the SanDisk PRO-G40 comes available in the finder (USB connection, Up to 10 Gb/s)
  10. Eject the disk properly (right-click menu)
  11. Reconnect the disk to the Mac Studio M1 Max using a Thunderbolt 4 from the back : the SanDisk PRO-G40 comes available in the finder (Thunderbolt 3 Device connected, Up to 40Gb/s)


Second test ... tying to repeat the same scenario ...

  1. Eject the disk properly (right-click menu) from the MacStudio
  2. Connect to the MacBook Pro : SanDisk PRO-G40 not available in the finder (but in the "Thunderbolt/USB4 Device Tree" of the System Information as "Device connected" at up to 40Gb/s; not available as "Storage")
  3. Disconnect
  4. Reconnect the disk to the Mac Studio M1 Max using a Thunderbolt 4 from the back : the SanDisk PRO-G40 comes available in the finder ...


Very strange. I cannot find any logic in this phenomenon.


What is needed for an external disk to become available as "Storage" in the Finder when it is listed as "Device connected" under Thunderbolt ?


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User profile for user: DominiekC
DominiekC Author
User level: Level 1
4 points

Sep 3, 2024 6:57 AM in response to Grant Bennet-Alder

Thank you for your information.


But how should I interpret that ?

Does it mean that a Thunderbolt 3 storage device is not at all compatible with a Thunderbolt 4 port, or does it mean that I should use a Thunderbolt 3 cable anyway and definitely not a Thunderbolt 4 cable and that such a Thunderbolt 3 cable can then still make a correct connection to the Thunderbolt 4 port to make a Thunderbolt 3 storage device work ?

In other words, is it mainly a cable version problem or also a port version problem ?


BTW : the SanDisk PRO-G40 external SSD drive came with a short Thunderbolt cable where a "3" is shown next to the lightning icon. My first (failed) testing was using that original cable and it was/is working on my MacStudio M1 Max Thunderbolt 4 ports on the back.

Could there be a difference between the Thunderbolt 4 ports on the back of a MacStudion M1 Max and the Thunderbolt 4 ports on a MacBook Pro M3 Pro ?


This is the System information on my MacStudio M1 Max when connected to a Thunderbolt 4 port on the back using the original short Thunderbolt 3 cable delivered with the device :



After a right click "Eject" on the MacStudio, I use the same cable to connect the device with the MacBook Pro M3 Pro and this is what I get in the system information :



But no external device in the "Storage" list nor the Finder !


After disconnecting (of course right click not available in the Finder) and reconnect to the same (back) port as before on the MacStudio, I do NOT get a reconnection as storage (only the Thunderbolt connection) and the drive is NOT available in the Finder ...

After disconnecting from the Thunderbolt port on the back of the MacStudio en reconnecting to one of the USB4 ports on the front of the MacStudio, I get the storage connected and available in the finder (of course at low speed). After a right click "Eject" in the finder and a reconnect to the Thunderbolt port on the back of the MacStudio, I get the storage available as Thunderbolt 3 drive as it was before.


It seems that a USB4 connection and "Eject" initiates a kind of "reset" of the failing device connections ...

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User profile for user: DominiekC
DominiekC Author
User level: Level 1
4 points

Aug 27, 2024 8:04 AM in response to Grant Bennet-Alder

Thank you for your reply.


The SSD drive was reformatted (APFS encrypted) on the MacStudio (using the MacOS Disk Utility software). In order to check an encryption related issue, I've already decrypted the drive, but this didn't solve the issue.


According the Western Digital website there is no additional software required for a Western Digital drive to function as storage device (drivers are delivered and installed by the macOS updates).


BTW : on the MacBook Pro M3 Pro the drive is also not detected by the MacOS Disk Utility software ...

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User profile for user: DominiekC
DominiekC Author
User level: Level 1
4 points

Aug 28, 2024 6:35 AM in response to DominiekC

I've done some additional testing using a Samsung Portable SSD T3 connected by an Anker Thunderbolt 4 cable to the MacBook Pro M3 Pro. This SSD is available as a storage device and uses the USB protocol.


In my work-around using an Anker 555 USB-C Hub and connect the SanDisk PRO-G40 external SSD through one of the USB-C ports on the Hub, the main connection is a also a USB connection ...


So my missing storage device in the Finder with the SanDisk PRO-G40 external SSD is an issue specific for the Thunderbolt/USB4 ports on the MacBook Pro M3 Pro when used as Thunderbolt port with a storage device using the PCI-Express protocol on Thunderbolt 3.


But still no solution to use this storage device on full speed (40Gb/s) on my MacBook Pro M3 Pro as I can with the same device and connection cable on my MacStudio M1 Max ...

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User profile for user: Grant Bennet-Alder
Grant Bennet-Alder
User level: Level 10
133,183 points

Aug 28, 2024 8:46 AM in response to DominiekC

(this is tangential, but not completely beside the point)


Regarding displays, we are seeing a rash of displays that do not connect properly on the newest Macs with the newest MacOS on ThunderBolt connections, unless they are using the beats cables.


The solution has been to make absolutely certain that the cables involved have the ThunderBolt symbol on each end (indicating certified ThunderBolt cables) and that the cable length does not exceed 0.8 meters for genuine Apple cables, and 0.5 meters for brand-X cables.


A cable like that may be worth a try, to be certain the high data rates and cable losses are not causing it to fall back to USB speeds.


Please let the community know what you learn if you do some experiments.

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User profile for user: Grant Bennet-Alder
Grant Bennet-Alder
User level: Level 10
133,183 points

Sep 2, 2024 11:53 AM in response to DominiekC

By far the easiest way to cause poor performance, instability, overheating and crashing is to install ANY third-party speeder-uppers, Cleaners, Optimizers, or Virus scanners, Bit Torrent, or a VPN that you installed yourself. They are relentless in scanning your files, non-stop, looking for virus-like patterns in Everything, or looking for files that have changed. When completed, they do it all again.


¿Are you running ANYTHING in any of those categories?

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User profile for user: Grant Bennet-Alder
Grant Bennet-Alder
User level: Level 10
133,183 points

Sep 2, 2024 2:33 PM in response to DominiekC

Your drives are ThunderBolt-3. To use those drives, you may need to follow the rules for Thunderbolt-3, because the drives do not have Driver circuitry used in Thunderbolt-4 that can reliably drive a meter of cable.


I suggest you obtain a 0.5 meter Thunderbolt-3 cable, or if Apple brand 0.8 meters, and run your experiments again.

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User profile for user: Grant Bennet-Alder
Grant Bennet-Alder
User level: Level 10
133,183 points

Sep 2, 2024 5:30 PM in response to Grant Bennet-Alder

I have now confirmed that ThunderBolt-4 obtains its higher data rates, better noise immunity, and longer allowed cable lengths by COMPLETELY changing the signaling method and using a COMPLETELY different modulation scheme, which is not readable by Thunderbolt-3 devices. Its underlying carrier rate is actually slower, and it encodes multiple bits per signaling interval.


Movie version: older devices can't talk native Thunderbolt-4.

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User profile for user: Grant Bennet-Alder
Grant Bennet-Alder
User level: Level 10
133,183 points

Sep 3, 2024 7:12 AM in response to DominiekC

the front port on a Mac Studio are "only" 10 G bits sec USB ports, unless you have the ULTRA processor. So your reported front-port behavior is completely expected.


<< My first (failed) testing was using that original cable and it was/is working on my MacStudio M1 Max Thunderbolt 4 ports on the back. >>


If you are not willing to obtain a very highest spec Thunderbolt-3 cable (which is limited in length to 0.8 meter for Apple cables and 0.5 meters for brand-X cables) THEN:


... you should contact the drive-maker and tell them the COMBINATION of their drive WITH THEIR ThunderBolt-3 CABLE is not performing adequately. they can read what you have already posted on these public forums, if they are wiling.


My experience with drive support folks form different drive makers is that they try to make sure you have it plugged in, then send you a different drive (and cable if you insist).


ThunderBolt-4 gets it increased maximum cable-length by using different signaling on the cable, which only works with genuine ThunderBolt-4 devices. You need the shorter cable for top speed on Thunderbolt-3 devices.

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Macbook Pro M3 Pro : External SSD with Thunderbolt 3 port not available in Finder

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