M4 Mac Mini - External Thunderbolt Drives Limitation

[Sarpanch]

I have a M4 Mac Mini and ran into a strange limitation that wouldn’t allow 3 external bus-powered Thunderbolt SSDs to be connected to the Thunderbolt 4 ports. 2 drives work great, but the 3rd is not even recognized by the system. The issue is not specific to any of the 3 ports or drives, it’s always the last one that fails to connect. Surprisingly, 2 Thunderbolt drives + 1 USB-C drive work okay, which indicates that the Mac Mini might be downgrading the 3rd port to USB-C.

Does anyone know what could be the reason? It’s a bit misleading to advertise 3 TB4 ports if only 2 are fully functional concurrently.

Additional reports that I could find are linked below:

https://www.reddit.com/r/macmini/comments/1gprem6/m4_mac_mini_are_you_able_to_connect_3_thunderbolt

Why can't I use all 4 Thunderbolt ports o… - Apple Community

discussions.apple.com

 

[BrianBaughn]

I can't say for sure but it sounds like Apple has done the same BS that's in the M1 Mac Studio.

https://forums.macrumors.com/threads/my-mac-studio-only-recognizes-my-thunderbolt-ssd-enclosure-as-usb-3-1-and-not-as-a-thunderbolt-device-update.2380985/

 

[blufrog]

Could be a port power issue, and that 3xTB devices are drawing too much power, so it is degrading the interface.

My guess it is negotiating TB, failing on power, re-negotiating USB, succeeding, and so you don't get the power message. If the device was TB only and could not fall back to USB, I'll bet you'd get the insufficient port power message.

Studio Ultra - Simultaneous use of Bus Powered TB devices limited? (title edited)

Today I discovered that I cannot connect more then three thunderbolt drives (and the BenQ display using USBC) to my Studio Ultra. On the back four ports, I have connected: 1. BenQ SW321C 2. ProGrade TB3 CF Express card reader 3. & 4. Orico TB4 enclosure w/Samsung 980 Pro NVMe SSD If I connect...

forums.macrumors.com

 

[BrianBaughn]

My 4K LG monitor (which, of course, has its own power supply) counts as a TB device as far as my M1 Mac Studio is concerned. I get the monitor and one TB external drive working when I plug the monitor and two TB external drives (in Acasis TBU405 Air enclosures) into three of the four ports on the back. I don't get any system message about the drive that doesn't connect.

In my case I got a Kensington TB4 hub to get all three working.

 

[Ray2]

Other than for travel, I only buy drives with dedicated power supplies. I’d rather blow one of those, a sub-$15 replacement, than blow the power supply in my computer. 3 bus powered drives at the same time, no thanks.

 

[MacCheetah3]

Could be a port power issue, and that 3xTB devices are drawing too much power, so it is degrading the interface.

In my setup it is. And I’m wondering if not all three Thunderbolt ports support a full 15W. Maybe it’s just due to power consumption spikes? Anyway… I have two USB 3.1 (USB-C) NVMe enclosures and a USB 3.1 (USB-A) 2.5-inch drive connected via USB4 hub to one rear port. The USB-A drive is only on/powered briefly as it's my once a week Time Machine backup — I do have other backups. I have one OWC 1M2 connected to another of the Thunderbolt ports and recently added a second 1M2, which I connected to the last available rear port. When I have all (four) of them connected (mounted or not), the USB 3.1 drives will randomly disconnect (i.e., “Disk not ejected properly” warning). The USB over current alert did appear once or twice though not every unexpected unmounting instance. Each of the USB 3.1 drives is reportedly using its ~900mA allotment. I can’t find power information for the Thunderbolt (1M2) drives.

So, as of now, I connect the 1M2s as needed. Inconvenient indeed but better than drives dropping out randomly. Maybe I’ll eventually buy a powered hub for the USB 3.1 devices.

 

[PaulD-UK]

@MacCheetah3 The problem is exacerbated by using the USB4 hub, as all 3 USB devices attached to it are controlled by the USB controller in the Mac.
If it was a Thunderbolt 3 dock then all attached drives would be controlled by the dock's USB controller, and powered from the dock.
I know that's probably a different user experience than what you want/have, but that works around the Mac's sensitivity issues to USB attached/controlled SSDs/HDs.

I also suspect that the instantaneous 'inrush current' for USB4 bus-powered SSDs may be the reason there is a limit to using bus-powered devices on all the TB4 ports...
Another AS Mac 'sensitivity'...?

 

[MacCheetah3]

@MacCheetah3 The problem is exacerbated by using the USB4 hub, as all 3 USB devices attached to it are controlled by the USB controller in the Mac.
If it was a Thunderbolt 3 dock then all attached drives would be controlled by the dock's USB controller, and powered from the dock.
I know that's probably a different user experience than what you want/have, but that works around the Mac's sensitivity issues to USB attached/controlled SSDs/HDs.

The problem (I have) with a “dock” is they are overkill for my needs. I likely wouldn’t use even half of the ports they provide. So, the additional size and cost is just wasteful.

Something like this probably would have been a better choice as it does allow for external power:

OWC 5 Port Thunderbolt Hub - 60W Charging, (3) Thunderbolt 4 (USB-C) Ports, (1) USB 3.2 Gen 2 Type-A Port, Dual Display suppor, Compatible with Macs and PC's

Actual display support varies upon computer specifications. Supported Intel-based Mac models and Thunderbolt 4 PCs support two displays over a Thunderbolt port. The following Apple M1 Mac models support a single display over a Thunderbolt port: - MacBook Pro (13-inch, M1, 2020) - MacBookPro17,1 -...

www.amazon.com

I was trying to keep the cost and bulk sensibly low.

I could also connect one of the USB 3 drives to one of the front ports — assuming that would bypass the problem/limit.

 

[PaulD-UK]

@MacCheetah3 The problem with that OWC hub (which is actually a dock but with multiple TB4 output ports in 'hub' configuration) is that because it uses the TB4 protocol it doesn't have a USB controller, as it allows the Mac’s USB Host controller to 'tunnel' through to its ports.
So any problems the Mac perceives with attached USB 3.* SSDs still causes the disconnections, errors etc
Edit: (It would work to provide power for the USB4 enclosures, but more than one would have to share bandwidth if used simultaneously)

But for USB3.* SSDs it’s only in a TB3 dock that the Mac passes through PCIe signals to a USB controller in the dock, and because the Mac isn't communicating directly to those USB devices attached to the dock, it doesn't see any problems, so everything is stable and remains connected (with well-functioning devices...)

 

[Sarpanch]

Thanks everyone, for sharing a lot of valuable info. I spoke with Apple support and one of the techs hinted at a power draw limitation in the board itself. He could not explain why, nor confirm if this limitation exists on the new Thunderbolt 5 Macs or not.

Second, I just got a Thunderbolt Display (Dell U2724DE) and can confirm that the Mac Mini works just fine with the monitor + 2 external bus powered thunderbolt SSDs connected directly to the Mini’s T4 ports + 1 external thunderbolt SSD connected to the monitor. The write speed marginally reduces on the SSD connected to the monitor (~2100 MB/s) but it’s plenty fast for my use case