[Server-devel] Western Digital hard disk data

[George Hunt]

Hi all,

Following James' suggestion, I put my fluke 186 multimeter in current mode
on the USB cable between the power source and the WD hard disk.  I was able
to capture a 1A current spike, and a ~ 3 second current transient of about
800mA during spin up.  The idling current for the hard disk, while
spinning, was 230mA.

The XO4, (sku306) did not reliably spin up the HD. In particular this
failure was almost 100% when the 430mA load of the ethernet dongle, and
MR3020 wifi access point was hanging on the other USB port.

I put the hard drive and wifi accessories on a powered "Plugable" model
USB2HUB AG7, which has an adapter rated for 3A, and experienced no
failures, during many startups.

I am concluding that an external hard disk requires a powered USB hub.

Other interesting measurements:

   - The MR3020 wifi unit had maximum current spikes of 530mA with average
   current of 175mA.
   - The USB ethernet dongle had 230mA spikes with an average of 177mA and
   a minimum of 172mA


George
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