Mini-USB cable will only do data, but charges other devices?

[creedo]

I guess the obvious answer is "get a different cable" but I'm puzzled why this should be.

This cable passes data and also powers my external hard drive.
With my TP2 it passes data but won't charge.

Other generic mini-usb's will charge this phone, why is this one being fussy? It's the correct shape/format (not the smaller micro-usb that you see with blackberries etc). The only thing that's funny about it, is it doesn't go in all the way to the "hilt" like most mini usb cables.

edit: Fixed it... stupid software issue. I just noticed my usual battery icon showed 0% though my other battery icon (rainbow bar at the top) showed like 40%. Clicked the 0% icon, got to notifications screen, clicked the notifications screen button next to battery, got a checkbox I'd never bothered with before that said something like "don't charge this phone while it's attached to a PC". Not sure why you'd ever want to do that.

Unchecked it, got an error about how I'm using an unsuitable charging device and should charge with manufacturer's charger etc. Unplugged and replugged, now charging normally without errors.
<3 WM