USB HD operation without external power?

Do you have a question? Post it now! No Registration Necessary.  Now with pictures!

Threaded View
I have a WD Passport 80gb USB external hard drive.  I've always
faithfully plugged in its power supply before using it and it's always
worked great.  Yesterday, I plugged in the USB cord first and darned
if it didn't start up and I was able to access and use it just fine!
Now I wonder if I need to use the external power supply at all.  Does
powering the drive VIA the USB port only dammage anything?  Is data
transfer compromised in some way?

Thanks very much.

Re: USB HD operation without external power?

Dave Smith wrote:
Quoted text here. Click to load it

Well, this has got to be a 2.5" drive. A 3.5" could not move
without its external supply.

A 2.5" drive needs 5V @ 1A max to run (which is why some 2.5" products
use a "dual headed" USB cable for power). A USB port is only
supposed to provide 5V @ 500mA. If you plugged the drive
into a laptop, chances are the laptop would have a fancier
overcurrent detection feature. Some desktop boards only have
a Polyfuse, and I think the fuse is set to above 1A.
(That is because the Polyfuse is shared by two USB
ports in a port stack, to save money.)

To say more, you'd need to download a spec for the hard drive
inside the enclosure.

I looked here, but they don't state what the spinup current is.

This FAQ entry says the power supply is optional. Maybe it is
intended for laptops, or other equipment where the overcurrent
thing gets tripped. The spec here says max current is 5V @ 650mA
at the drive level. That, in theory, should be too much for one
USB port, if enforced rigidly by the computer. The 650mA would
only be drawn for 10 seconds, until the drive gets up to speed.
Once up to speed, the biggest power draw is 500mA, during a
write operation (that number is from the spec page above).

    External Power Specifications (optional)
    AC input voltage   5.0 VDC, 1.0A
    Drive Maximum Power Draw (milliamps)   650 Ma

As long as the drive spins up, and you are able to
access it, I'd say you're fine. If it was my drive,
I'd plug it in :-) Because that is the kind of guy
I am (not a risk taker).


Re: USB HD operation without external power?

Quoted text here. Click to load it

is there a thing (standard thing?) he can get that plugs into 2 USB
ports and provides 5V 1A for USB devices?

Is it possible and easy to build such a thing ?

Maybe taking the red-VCC/5V , and, black-GND wires of the USB cable.
Get some more wires similar or same thickness. Connect 2 wires to the
red, 2 wires to the black, pref same colours for ease.

Can leave the 2 data wires, don't split them.. They aren't even used
at all anyway.

Connect a pair of red and black to the end of one USB cable..
Connect the other pair of red and black to the end of another USB

Hey presto.

I don't know if that will give 5V 1A from 2* 5V 500mA ports though!?

If so, then hopefully somebody's already done it, would save some

The only time I chose to molest a USB cable, was 'cos I had a car
charger that had a 5V USB cable, and I wanted to replace the USB
connector with some weird connector for my HP IPAQ that was also 5V.
(I've since found some car chargers for the IPAQ that don't make a
high pitched noise, so I use those!). So, maybe this is another thing
to put in the category of useful / innovative ways to molest a USB

Re: USB HD operation without external power? wrote:

Quoted text here. Click to load it

The solutions already come with 2.5" enclosures, so you don't have
to build your own.

This solution uses two USB cables, and puts a power connector on
the end of one of them. I suppose the benefit of this is, you could
connect a wall wart power source in place of the second cable.

This one is a Y cable. Which means the 5V @ 1A is flowing through the
single VCC pin on the other end of the Y cable. I don't know what the
max current rating is for a USB pin, and whether the pin is rated
for that kind of current.

I cannot say I'm that crazy about some of the things that could happen,
using concepts like those two. For example, on an older motherboard
that has headers for setting the power source for the USB stacks,
you could have one stack powered by +5VSB, one stack powered by +5V,
use either of the above solutions, and short +5V to +5VSB. Then the
user cannot figure out why the computer is suddenly acting funny. But
seeing as the enclosures are $10-$20 items, it is not surprising they
do stuff like that. I'd much prefer a "normal" power source, and
a single USB cable, if the enclosure was something I was buying.


Re: USB HD operation without external power?

On 25 May 2007 04:58:20 -0700, ""

Quoted text here. Click to load it

Probably easiest to just put a powered hub in series with
them, one with suitably high capacity external supply then
IF it ever blows the fuse, put a slightly higher rated fuse
in it's place.

Re: USB HD operation without external power?

kony wrote:
Quoted text here. Click to load it

Doesn't that reduce the speed?

                        cbfalconer at maineline dot net

Posted via a free Usenet account from

Re: USB HD operation without external power?

On Fri, 25 May 2007 16:41:50 -0400, CBFalconer

Quoted text here. Click to load it

Negligably so if using only the one device per hub that
would have been used on the USB port-pair so there is no
bandwidth sharing for concurrent connected device access.

Re: USB HD operation without external power?

Quoted text here. Click to load it

It's better to plug it in, some cheepie usb controllers can't pull a
large power draw long term. Of course it's entirely possible you'll be

Re: USB HD operation without external power?


Quoted text here. Click to load it

Probably not. If the drive worked fine, then chances are it will
continue to do so. Many modern 2.5" (laptop) hard drives have dropped
their power consumption under or to the 500mA current limit on most
USB ports.

Do you know the RPM of the hard drive that is inside the external
case? 4200 and 5400 RPM drives should be pretty safe to run from a USB
port. A 7200 RPM drive still draws a lot of power and should be run
from the power adapter if you have any question about the maximum
current available from your USB ports.

Some USB ports will stand having much more current drawn from them
than normal. I have a 7200 RPM 100GB Hitachi hard drive in an Other
World Computing Mercury Elite On-The-Go enclosure. So far only one
computer--an IBM NetVista 8364--has refused to power it. At least six
other systems and one powered USB hub will run it just fine. This
drive has run for days attached to a USB port, without spinning down.

Hitachi shows the power draw to be 1.1 amps from this drive.


Re: USB HD operation without external power?


Quoted text here. Click to load it

Thanks very much to everyone who posted.  If I remember correctly, it
is a 5400RPM 2.5 inch drive.  I guess I'll do some experimenting as
long as nothing will be damaged.

Site Timeline