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.

Dave Smith wrote:
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.

http://www.westerndigital.com/en/pro...uage=en#jump88

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).

http://wdc.custhelp.com/cgi-bin/wdc....p?p_faqid=1481

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).

HTH,
Paul

On May 21, 7:03 pm, Dave Smith <Sally... (AT) cox (DOT) net> wrote:
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
fine.

Hi!

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.

William