Will a single USB 3.0 port have sufficient power (via a non-powered hub) to run two external 2.5" platter hard drives (the type that don't require an AC power adapter)?

The hub is a Sabrent 4-Port USB 3.0 Hub with Individual Power Switches and LEDs (HB-UM43).

The hard drives are Western Digital "My Passport" 1TB USB 3.0 drives (several years old), product # WDBBEP0010BBK-01. Plugging either drive into a USB 3.0 port, does not show any power consumption via Win7's Device Manager (strange). Plugging a drive into a USB 2.0 port shows 500ma. The labels on the hard drives do not reveal power consumption information.

  • Related: superuser.com/questions/309088/… – Ramhound Apr 26 '18 at 22:29
  • These drives are USB 2.0 compatible, which means, a single USB 2.0 port can power them. I don’t know offhand the power limitations of USB 3.0 vs USB 2.0 but I suspect they are large enough to support 2 drives. Now that might be all you can power before Windows complains about USB resources. – Ramhound Apr 26 '18 at 22:49
  • @Ramhound I was just reading reviews for the hub on Amazon (IOW, questionable validity of information), and one person is claiming that running a hard drive with too little power can damage the hard drive. Any truth to that? Worth a separate question? – RockPaperLizard Apr 26 '18 at 22:53
  • Either look at the label for power usages or go to the USB ports in the device manager to see how many ma they are drawing. Add them up, if the total is >900ma usb 3.0 is out. Certain usb 3.1 ports can provide 100w enough to easily power everything. However, the normal 3.1 limit is much lower, but still around 3000-4000ma, which would still work. – cybernard Apr 26 '18 at 22:54

If you had some ultra low power mechanical drives (I'm not sure they exist), you could, but not two of your drives or any I'm aware of.

The hub, itself, uses some power, then the drives would have to share what's left from the 900 ma connection to the laptop. At 500 ma for each drive, you would exceed what's available.

This would not work even with a special laptop charging port designed to deliver high charging current to an attached device. USB power is in "unit loads" (each unit load for USB 3.0 is 150 ma). USB devices have to negotiate for power beyond the first unit load. Hubs make a standard USB connection, which carries a maximum of 6 unit loads (900 ma) for USB 3.0 drawn from the laptop's port. USB 2.0 has a unit load of 100 ma, and a maximum of five, for a 500 ma limit.

With a powered hub, you could connect both of your drives. The combined data transfer rate for two USB 3.0 mechanical hard drives should still be well within the bandwidth of the hub's USB 3.0 connection.

  • 1
    Wow. Excellent answer. Thank you. When you say "would not work even with a charging port", do you mean a special type of USB port, or a powered hub? Would this work with a powered hub? – RockPaperLizard Apr 27 '18 at 1:37
  • 1
    I was referring to a special port on the laptop designed to provide more than standard current for device charging purposes. Any device wanting to use that high current would need to negotiate for it, and hubs don't do that. If you use a powered hub, that would work. Each hub port could deliver the standard USB maximum current, which would be enough for each drive. – fixer1234 Apr 27 '18 at 1:42
  • 1
    USB specifications and PD (power delivery) specifications are nearly independent, they are intentionally designed that way. As result, USB descriptor framework has no fields to refer to any extra power negotiations or any information about them, the PD is totally aside of USB. Thus there is no mentioning of any extra power that a hub may or may nor have, other than "self-powered" versus "bus-powered". Since the specifications are independent, no one prevents a design where a hub would negotiate PD from upstream port, and provide even more power as another downstream charging port... – Ale..chenski Aug 25 '18 at 20:23
  • 1
    To engage into PD, currently no standard means/interfaces are defined AFAIK, the PD is usually managed via additional MCU using I2C interface, all in proprietary manner. See Microsoft doc: docs.microsoft.com/en-us/windows-hardware/drivers/usbcon/… – Ale..chenski Aug 25 '18 at 20:25
  • 1
    @sampablokuper, I did find what I was thinking of, but it dated to before PD. At that time, hubs needed to have their own power supply to provide anything above the power from a standard connection. See Ali Chen's comments about PD. It looks like nothing precludes it, but it would be proprietary, so limited applicability. – fixer1234 Aug 25 '18 at 21:24

Just chipping in here, in case anyone other than me is looking to connect a bunch of bus-powered USB 3.0 harddrives to a system.

I am looking to connect 10 4TB WD 2.5" harddrives to an older computer. The machine doesn't even have USB 3, so I have to add an expansion card as well, in the single available PCIe slot there is (this is an older Lenovo workstation with just two PICe slots).

I've actually spent quite some time on this, because adding ten bus-powered drives will draw quite a bit of bus power. I haven't been able to find specific data on the drives I've chosen (Western Digital WDBJRT0040BBK-WESN ("WD Elements SE")), so I looked for other 2.5" WD drives, and found the specs for this this WD Blue drive, which lists the power consumption in different operation modes.

Interestingly, the "spin-up" power draw isn't explicitly listed (as it is on the front of most, if not all, of my 3.5" desktop drives. But, a "peak" power draw is listed, which has to be the spin-up, because this is by far when drives draw the most power (like accelerating a car from 0 to target speed).

This peak power is listed as 1A at 5V +/- 10%. The other power values are listed in Watts, but this value is easily translated, as Watts are simply V*A, so the peak is 5W +/- 10%, i.e. 4.5~5.5W peak (or 0.9~1.1 Amps at 5V).

You can see that the peak power draw is much higher than any of the "operational" modes, or in other words, the modes while spinning, either "read/write" or "idle". You have to make sure you have hubs or I/O cards that can support the peaks, because this is the power that is drawn when you turn the system on, or bring a drive from standby/sleep to running.

Okay, so according to the WD Blue specs, if this drive were put in a USB HDD enclosure as-is, it would at peak draw more than the 1000 mA (1A) peak of the drive itself, because you also have to power the USB logic board, probably an LED, and account for a bit of power loss as well. That is, unless there's some firmware fiddling going on, such as letting the drive spin up slower so as to stay within the USB 3.0 specs. The USB 3.0 specs dictate a maximum of 0.9A of current to be drawn from any port.

Obviously, the drive in the enclosure I'm getting is not this WD Blue drive, but given its power specs which would exceed the USB 3.0 specs, I conclude that it would be foolish of me not to assume that no matter which bus-powered drive I attach, it may draw up to the max current allowed by the USB 3.0 specs, i.e. 0.9A, or 5*0.9=4.5W.

The first conclusion from this is that no hub without a dedicated power supply will suffice for more than one drive. And even with just one drive, a bus-powered hub might not be able to reliably power, say, a keyboard, mouse or USB stick while an HDD is spinning up using its power.

So, any bus-powered hub is off-limits to me.

But, worse than this, if you're really looking to plug a hub full of HDDs, you have to make sure the hubs are quite beefy, power-wise, and this disqualifies a surprisingly large amount of hubs out there, even some from reputable brands that are quite expensive.

E.g., if you buy a 7-port hub, you would need one that can provide 7*0.9A of current at 5V, or 6.3A (31.5W). I dare you to find one (at anything resembling a reasonable price). The interwebs is ripe with bad USB hubs. I want something price-effective as well, so I initially went searching on AliExpress, but man, even if they look like they're good (e.g. Orico), and their reviews are great (on AliExpress, where I think the reviews are generally unreliable, skewed towards too-positive, due to the way their reward system is (was, they kinda fixed this) set up), when you start to search the greater webs and find them on, say, Amazon, you get to see the really really bad reviews.

So, I abandoned the cheap cheap options, and looked at getting something cheap, but from a brand I personally know not to disappoint. From the local retailers in Denmark, DeLock and LogiLink are brands that I think aren't really "brands" as such, but pickers of cheap hardware where they have a proper QA resulting in reliable hardware.

For the PCIe USB 3.0 adapter, I've chosen the cheap LogiLink PC0057A card, which states that it will deliver the USB 3.0-specified 0.9A per port, if I read the text correctly. And even if I don't, as you'll see, I'll use the power on only two ports, so chances are it'll work, especially given the fact that it takes a Molex connector for providing extra power.

It uses the VIA VL805 chipset, which AFAICT works out-of-the-box in Windows, Linux, and macOS, and I'm building a Hackintosh, so this is important to me. It also appears to be a chipset used by motherboard vendors, so my expectation is that it is reliable.

For the hubs, I've chosen two LogiLink UA0170 4-port hubs. I found also a nice 8-port LogiLink hub in aluminum and all that, but its power adapter delivers 2A at 12V, so 24W, or 3W per port max (assuming its power distribution is properly designed (it has a "fast-charge port")), or 0.6A, which is not enough to support every port simultaneously drawing max power, and ALSO NOT ENOUGH TO COMPLY TO SPECS, SO WTF :D.

The 4-port hubs I've chosen, though, come with 5V 3.5A PSUs, which still don't supply quite enough power — we need 0.9*4=3.6A, but it is darned close, so I'm banking on either luck here, or good capacitors in the hubs.

Good thing here is, I have four ports on the PCIe card, two of them will power and connect the first two drives, the last two ports will go to the hubs which will power and connect the last eight drives.

But, even if it turns out that the two hubs with their 3.5A on paper start complaining when I draw 3.6A of 5V goodness, I can still add a third 4-port hub and then put three drives on each of the now three hubs, the 10th drive on the last usb port on the card itself, leaving three ports open, one on each of the hubs, for other stuff.

Note that I haven't actually received any of this stuff yet, I've just finally ordered it, so I'm only talking from investigation, not actual experience :)

I will update this with my actual experience if I remember it. If I forget and you want it, comment and I'll remember :D

UPDATE: The PCIe card that I ordered was absolute crap. The internal Molex connector came soldered on at a tilted angle, protruding from the PCB. I removed it and attached a Molex cable instead. Then, the actual stability proved to be absolutely horrendous. Restarts would leave it not recognizing the devices attached to it, and you'd have to plug and unplug cables for five, ten or more minutes for it to finally register everything. Terrible. I finally gave up on it, and switched to a laptop with USB-C ports which I bought Ugreen USB 3.0 hubs for, and everything worked fine. Also worked fine on a 2014 Mac mini with 4 USB 3.0 ports on the back.

The hubs are so-so. After about a week, one PSU died. I got a replacement, on now, several months in, the remaining two PSUs are still humming along, having no problem driving four drives each.


Others have left an excellent answer to your question. I will answer a slightly different question - "How can I attach my two different HDDs to a single USB port". This is how you can supply the additional power needed.

Your two hard drives are Western Digital "My Passport" drives, which are just a WD branded HDD enclosure with a standard 2.5" WD hard disk inside. This particular enclosure can only be powered by the USB port.

One option is to use a USB Y cable, two examples are here and here (not endorsements). These cables have an extra Type-A connector for supplementing the power supply - the extra connector draws power from its USB port but doesn't have a data connection. You'd put this extra connector into a spare port somewhere or a wall powered USB hub.

Example USB Y cable picture

Another option is to remove the drives from the enclosure (the enclosures are old so you don't care about warranty) and put them in a USB hard disk enclosure which has its own 5V or 12V power source.


500ma x 2 =1000ma hard drives

500ma (worst case, but probably 250ma) hub

1500ma max

Normally the 3.0 USB port provides 900ma, in that case you are short 200-600ma. However, some motherboards have special 3.0 charging ports which can be up to 2.4A in which case it would power it no problem.

Get the manual from your motherboard

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.