On Wed, 9 Jan 2019 06:27:48 +0000 (UTC), Fred Smith
 wrote:

>On 2019-01-08, A  Dumas  wrote:
>> Vince Coen wrote:
>>> Any one have any recommendations as my brain is hurting ?
>>
>> Simply get any 2.5" portable disk. It won't need external power. Make sure
>> you use the official RPi power adapter for the Pi, though.
>>
>
> Even then I'd be using an externally, separately powered disk. The
> pi simply does not have the grunt to run "any 2.5" portable disk"
> reliably. You are asking for all sorts of obscure problems that
> will ruin your day.

I run a Raspberry Pi 3B with a 2.5" portable disk just fine.

Initially I had power problems, but those weren't solved by
using a second power adapter on a Y-cable to the disk. Instead,
that made it unstable, probably by "backfeeding".

The solution was to use a short (15 or 30 cm) USB power cable
with thick (18 AWG) power leads.

Search for "15cm 0.5FT Micro B USB Fast Quick Charge Cable 2.1A
Android SmartPhone 18AWG" on eBay or other sites. They are not
the cheapest, but work very well.

Most USB cables have too thin leads and are too long, resulting
in a considerable voltage drop. Also, the Pi itself only has
very small buffer capacitors, so peaks in power consumption
cause voltage drops, and the dreaded yellow thunderbolt on the
screen. With the short, thick power cables, the regulator in the
power adapter can follow the demand more closely.

In my opinion, even the official RPi power adapter is marginal.
Not because of its capacity, but the power cable could be
better.

--
Kind Regards,
Kees Nuyt

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