Inability of new 2025 4 port M4 iMac to connect directly to USB bus powered external back-up disks

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Level 1

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Inability of new 2025 4 port M4 iMac to connect directly to USB bus powered external back-up disks

Inability of new 2025 4 port M4 iMac to connect directly to USB bus powered external back-up (and other externally stored data) Hard Disks ...

I am disappointed to note that the fully optioned up M4 iMac (Sequoia 15.7.3, 10 core CPU, 10 core GPU, 16-core Neural Engine 32GB RAM, 2TB SSD) I purchased in mid-November 2025 appears to be unable to connect to external back-up (and other externally stored data) HDs that require 500mA power (which most ≥4TB 2.5" form factor external HDs now require).

Although, to be fair and as they say nowadays, for clarity and the avoidance of doubt, they will connect one time in twenty ... if you are lucky. But hardly a method to be relied upon for back-up.

These disks all work and connect flawlessly on four of the other Macs we own (and currently use, others dating back to my SE30 are in storage), mid-2010 17" MacBook Pro, 2x 2011 Mac mini, 2023 M3 Max 16" MacBook Pro.

I have tried different cables (new & old), but the only constant is that whilst the HDs requiring 100mA power connect every time (although they are too small <4TB for the job as required back-up), the ones requiring 500mA cannot be relied upon to connect.

As best as I now understand it, to resolve this problem, because, make no mistake, this is a problem, a powered USB-C hub must now be purchased to provide enough power so disks that have worked perfectly well on other Macs, can operate on the newest iteration of an iMac ... insert eye roll, shrug & face palm emojis here ...

Does anyone have a view on this that could resolve the issue without needing to clutter up the desk with extraneous hubs, wallwarts and cabling (like getting the latest iteration of a computer to ante up enough power to the USB bus?).

Posted on Dec 7, 2025 11:17 PM

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Answer 1

Dec 9, 2025 8:18 PM in response to NucMed

NucMed wrote:

I have tried different cables (new & old), but the only constant is that whilst the HDs requiring 100mA power connect every time (although they are too small <4TB for the job as required back-up), the ones requiring 500mA cannot be relied upon to connect.

An aside: If these are USB 3 drives, I'm surprised that they only want 500 mA.

USB 2 calls for a host port to provide up to 500 mA of 5V power (2.5W).
USB 3 calls for a host port to provide up to 900 mA of 5V power (4.5W).
When I plug large portable USB 3 hard drives into the front-panel ports on my Mac Studio, System Information often shows a single drive grabbing 896 mA of the 900 mA available!

This is why, if you use any sort of USB hub, you want it to have its own power supply. An unpowered hub may not have enough power for more than itself and a single drive.

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Answer 2

HWTech

Level 9

68,534 points

Dec 9, 2025 7:06 PM in response to NucMed

The USB3.x ports should provide up to 900mA. Check the System Profiler's USB section as it will clearly state the available power for that port when a device is connected to it and how much power that device is requesting. Actually with macOS 26.x Tahoe, it looks like Apple removed the "power available" portion from that location....another wonderful step backwards.

FYI, I have on occasion experienced issues connecting some drives & devices directly to the USB3 ports on a Mac and only using a powered USB3 hub allowed those devices to work properly...it was not even a power related issue....this was back in 2014. Plus I have seen numerous people on this forum confirm that their external drives will not work properly unless connected to a USB3 hub (not always a powered hub either).

There may be other issues at play here than a power related issue.

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Answer 3

den.thed

Level 10

90,850 points

Dec 8, 2025 8:17 AM in response to NucMed

NucMed wrote:

Does anyone have a view on this that could resolve the issue without needing to clutter up the desk with extraneous hubs, wallwarts and cabling (like getting the latest iteration of a computer to ante up enough power to the USB bus?).

The best way to avoid hub, adapter, cable and external drive clutter?

Is to replace them with a 4 bay drive enclosure.

https://www.owc.com/solutions/mercury-elite-pro-quad

Or replace them with a 4 slot NVMe M.2 enclosure.

https://eshop.macsales.com/item/OWC/US4EXP4M2/

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Answer 4

NucMed Author

Level 1

24 points

Dec 8, 2025 8:21 PM in response to den.thed

Whilst I understand your response is a literal one to a very defined part of my above vent, given that there are already four (4) ≥4TB 2.5" form factor external hard disks containing data (and a new 5TB 2.5" form factor external hard disk to be used as an additional back-up) the suggestion is to spend a not inconsiderable greater sum of money on top of the ≈ AU$4 500 for the iMac so it can connect and use hard disks containing data that has worked seamlessly on other Macs ranging from 2-15 years old? Even were those items, or adjacent, easily available to me, which they are currently, nor in the immediate (<3/12) future, not.

A bit risible when Apple say (as an excuse for only offering a paltry amount of internal storage, even BTO maxed out ...) "just plug in an external disk with all your data" (but then not provide the required bus power for such a system to work).

BTW, being in AU & KZ, we do not have such easy & cheap access to the linked above such items, although, were we starting absolutely from scratch with this and absolutely no legacy data already on storage devices, we *might* consider such a system.

As an aside for clarity and as they say nowadays, the avoidance of doubt and to forestall those that might say ... oh, just back it up to the "cloud" ... (Really? What, the whole >16TB? And you'd be happy to trust that?), not everyone has access to reliable (c/f just about any major recent system collapse ... hello AWS/Cloudflare/Crowdstrike/Google) fast internet and telecoms connections everywhere in the world, another curse being the bizarre obsession with repeated nag screens to enable TFA (and if you decline, having to go through four stages to log in) ... No, I'll have my data storage where I can physically hold it in my hand (and cycle it weekly off-site thank you very much) rather than in some tech-bro's fantasy-land in the sky.

The above is not an invitation to a pity-party, merely pointing out not everyone has access to, in *some* areas, the means to resolve a problem that appears to have manifested itself recently in at least one Apple product given all disks work correctly connected to my 16" M3 Max MacBook Pro 16‑Core CPU and 40‑Core GPU (8TB SSD 128GB RAM) model nr A2991.

Yes, I have given this feedback to Apple. And what is it with this repeated "your session has expired business" despite being logged in on a browser on my laptop? ... this as I've had to do with the original post, do it on a 'phone ...

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