[Ham-Computers] RE: External USB Hard Drive - Work on an OlderComputer?

[Hsu, Aaron (NBC Universal)]

As already stated earlier, USB drives are USB drives - they are not
"platform" specific and will work on desktops, laptops, servers, and
even non-computer devices (network storage adapters and digital cameras,
for example).

However, not all USB ports are created equal.  It is true that some
"stand alone" USB hard drives won't work with some USB ports, especially
ports on older PC's.  This is not due to a protocol problem - it's due
to a power problem.  The original USB spec states that USB ports are to
supply up to 500ma of current at +5 volts.  However, IIRC, there's no
stipulation in USB 1.1 that a port *must* supply 500ma...just *up to*
500ma.  I believe with USB 2.0, a port *must* be able to supply the full
500ma to be considered "in spec".

Most USB interface chips have over-current protection set to 500ma and
will usually disable the port if more than 500ma is drawn.  However, if
the port couldn't supply the full 500ma, then no warning would be given
and the device might run in a "marginal" state.  Some ports have
built-in current limiting, so you can't draw more than 500ma in which
case the device might not start up.

I've experienced these conditions with several 2.5" USB hard drives -
Windows would detect the USB-to-IDE bridge chip of the external case,
but would not be able to recognize the hard drive as it couldn't
initialize.  Adding an external power source, or using a special cable
that takes up two USB ports (effectively allowing upto 1000ma of current
for the device), usually solves this problem.

For reference, note that USB cabling might also play a factor here.
Many USB cables are made from 24 AWG wiring.  On longer cables, the
voltage drop across the wiring may be high enough to prevent a device
from working properly.  This is expecially true for power hungry devices
such as a hard disk drive.  I believe cables certified for "High Speed"
USB 2.0 are designed with larger AWG wire to handle both power and
bandwidth (480Mbps) requrements.  I do remember that the USB 2.0 spec
did include provisions for power requirements that weren't met with the
original USB spec.

One more thought...although most 2.5" hard drives today are designed
with a 500ma power spec, this may be exceeded during spin-up.  Ususally,
"boxed" solutions are more likely to work as the vendor has taken the
power requirements of the drive and the USB interface board into
consideration (and maybe even voltage drop across the included USB
cable).  This may mean that the drive runs at a lower performance spec
than "OEM" 2.5" drives designed for a laptop (such as 4200RPM vs 5400RPM
spin rate).  The "do it yourself" external 2.5" USB hard drive may not
work as certain combinations of external case and "OEM" drive may exceed
the power spec of the USB port - even more true on the USB ports of
earlier PC's.

73,

  - Aaron, NN6O





-----Original Message-----
Sent: Sunday, February 26, 2006 11:28 AM
Subject: Re: [Ham-Computers] External USB Hard Drive - Work on an
OlderComputer? 


Thanks for the replies. I wanted an external hard drive that would work
on 
a lap top and desk top computer, but this type drive is not available. I

went to BestBuy, and was told that they had 2.5" External drives for lap

top computers and 3.5" External drives for desk top computers, but
nothing 
for both.  I guess power requirements differ.
Jim

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