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For the most 2,5" external hdds (without extra power connection) i need a Micro-USB-B-to-USB-A-plug like this: enter image description here

I have not seen any other kind of devices which uses a Micro-USB-B-plug yet. Furthermore i can not find many external hdds without extra power connection which have not a Micro-USB-B-plug. So why are Micro-USB-B-plugs very common for this kind of devices but not for other kind of devices? Do the manufacturer have technical reasons to prefer Micro-USB-B?

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  • A short answer, it's cheap, supplies sufficient power for most drives, and provides wide compatibility with computers with USB being so common. Micro-USB-B offers at least 7.5 watts on any USB host, be it USB 2.0 or USB 3.x, enough for most any drive. With the right cable it can attach to hosts with any of a half dozen USB host port types. If the host is USB 3.x capable then all the needed pins are there. If the host is USB 2.0 then the smaller micro-USB-B plugs, without the SS pins, can plug in with cheap and ubiquitous cables. It comes down to cost and market demands.
    – MacGuffin
    Jan 5, 2021 at 8:50

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I think you are getting confused here. The cable in your picture has the USB 3 SuperSpeed variant of the Micro-B connector which looks different to the original version that most people recognise.

enter image description here

Original Micro-B (Wikipedia)

enter image description here

USB 3 SuperSpeed Micro-B (Wikipedia)

This USB 3 version introduced an additional section with extra pins allowing faster transfer speed and more current which can be useful for devices like external hard drives. Even a 2.5" traditional hard drive can often reach speeds over 100MB/s which is well beyond the practical speeds of USB 2.0. Additionally USB 2.0 is only required to supply up to 500mA is often too low for such devices.

But many other devices do not really need this extra power or bandwidth plus manufacturers were already developing workarounds to get past the original current limit of USB 2.0. Additionally the new connector can be fiddly and unreliable in practice given its odd shape.

So the standard USB 2.0 Micro-B connector is still being used quite a lot on small devices today.

Larger USB devices such as printers and 3.5" external hard drives have traditionally used full size USB B connectors since having a smaller connector would be unnecessary and less durable. One of the problems with the Micro-B connector is that it's too easy for people to try to force the cable in when it's the the wrong way up and the small size can sometimes make it a bit fragile.

The full size B was also modified with additional pins for the USB 3 specification.

Of course the USB-C connector is slowly becoming more widespread and is effectively replacing B and Micro-B due to it being superior in almost all respects.

Wikipedia has a useful article explaining the types of connectors

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I have not seen any other kind of devices which uses a Micro-USB-B-plug yet.

I'm curious, what other devices are you looking at? What kinds of connections are you seeing instead?

Furthermore i can not find many external hdds without extra power connection which have not a Micro-USB-B-plug. So why are Micro-USB-B-plugs very common for this kind of devices but not for other kind of devices?

I'm not sure I follow. If it has micro-USB-B then it doesn't have a separate power connector?

Micro-USB-B allows for a maximum of 12 watts of power to the device. If a device needs more than 12 watts then it's going to need a separate power connection or a connector that supports more power transfer, like USB-C.

Do the manufacturer have technical reasons to prefer Micro-USB-B?

Mostly it comes down to market demands and cost. Most people will want USB unless there is a specific need for something faster. Once it is decided to use USB then it comes to choosing which one of the USB connectors will work for the device at the lowest cost.

All mini-USB connectors are limited to USB 2.0 speeds, so if the device needs something faster then it's going to be the standard size USB-B, the micro sized connector, or USB-C. The standard size USB-B is quite large for something like a portable hard drive, and micro-USB-B is cheaper than USB-C.

I would not consider micro-USB-B to be all that rare or unusual. It wasn't that long ago when nearly every cell phone used micro-USB-B for power and data. Any desktop, non-portable, peripheral will likely use the larger (and likely cheaper) USB-B connector. With USB-C allowing for higher power transfer, a "flippable" connector, and more bandwidth, I can see why it's a popular alternative to micro-USB-B.

I believe what you are seeing is that USB-C is pushing micro-USB-B out of the market and portable hard drives are simply the last man standing. They rarely need more than 12 watts of power or more than 5 Gbps that micro-USB-B provides so putting in a more expensive USB-C connector is unnecessary.

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