Is it safe to use a micro-USB 2 cable in a USB 3 portable hard drive?
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The micro-B USB 3 port seems to be somewhat backwards compatible with the old micro-USB 2 standard. I was able to charge a phone that had the micro-B USB 3 port with a micro-USB 2 cable.
However, I am not sure to what extent this is a part of the USB standard or just something the phone manufacturer did for backwards compatibility.
Yes, it's safe. The base connector is identical to the original Micro-B, only adding the USB 3.0 data send/receive lines on the side. (USB 3.0 is backwards-compatible with 2.0, and this applies equally to all connectors it introduced – both the full-size and micro-size, type-A and type-B ports).
The end result will generally be the same as if connecting the drive to a computer's USB 2.0 port (which doesn't have these extra pins, either) – you'll be limited to the speed and power that USB 2.0 allows and won't be able to use UAS, but it should still work properly otherwise.
