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I am shopping for an AC/DC power adapter for an external hard drive, and I've noticed that the power supply required for the couple of the external hard drives I looked at is always with positive polarity. This made me wonder: Do all external hard drives require a power supply with positive polarity? In other words, if I buy AC/DC power adapter for an external hard drive, should its output always have positive polarity, or does that depend on the external hard drive?

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  • There is one 'self evident' answer to your question: Yes. Now whether to extend on that depends on what you're actually asking; it is somewhat fuzzy. I have yet to something differing from that, and I have been tinkering with computer hardware and other electronics for decades. (Not old UK -made cars though... ;-)
    – Hannu
    Commented Nov 23, 2023 at 6:06
  • @Hannu thanks, to rephrase: if I buy AC/DC power adapter for an external hard drive, should its output always have positive polarity, or does that depend on the external hard drive?
    – Franck Dernoncourt
    Commented Nov 23, 2023 at 6:23
  • If you find such a thing (that is directly connectable) it should be correctly wired. Now I'd recommend look for a USB caddy instead. => google.com/search?q=USB+HDD+caddy
    – Hannu
    Commented Nov 23, 2023 at 6:29
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    All electronics requires some kind of power source where one conductor is at a higher polarity than the other. A -12V supply can simply be considered as one where the more positive connection has been labelled as "0V" and the distinction of "positive" and "negative" supplies only becomes important where multiple supplies are used together with a common 0V or ground connection. Taken individually every power supply has an inherent positive terminal alongside a negative one. You probably need to clarify exactly what you are asking for.
    – Mokubai ♦
    Commented Nov 23, 2023 at 7:33
  • Are you asking about why power is on a particular pin? I.e. connector polarity. Or are you asking why it is a +12V supply rather than a -12V supply? As it stands your question is ambiguous.
    – Mokubai ♦
    Commented Nov 23, 2023 at 8:01

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Its more that negative center barrel plugs are less common and tend to be found (very rarely) in guitar pedals. Most barrel plugs are center negative and PSUs tend to follow that.

For safety's sake though it is always good to double check.

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