In terms of 2.5" external hard drives with a micro USB connection - I notice that if
I use a USB cable that is too long - the drive starts clicking as if the heads have failed.

However, the moment I use a shorter USB cable - the drive spins up normally?

Explain this one?

Correct power supply is not reaching the drive.

Ok, its a power issue. Thank you for clarifying that.

Why do some HDD's need 2 USB cables connected to the host system
whilst others only need 1?

Read this http://en.m.wikipedia.org/wiki/USB

Then check the specs of your HDD.

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All went well until I plugged the drive in.

Thanks.

USB 2.0 ports are current limited to 500mA. Most 2.5" HDDs require significantly more current (~850mA) during spin-up. Some USB ports are OK with this, but many aren't. For the latter, a Y-cable can be used to pick up power from two USB ports.

USB 3.0 can handle 900mA.

You can see a typical HDD startup current profile in most Seagate Momentus product manuals.

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A backup a day keeps DR away.

Thanks fzabkar

Hi,
get USBDeview from Nirsoft.
when you plug in your usb drive, run USBDeview and locate the listing for your drive and one of the columns reports what your drive is telling the system the required power in mW is.