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I've seen some external hdd-cases(for 2.5 inch) with two-head usb:

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However, all hdd's are working with 1 usb-head. I know that they are not enough to power 3.5inch hdds, so , what are use of two-usb heads?

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    Your answer is in your question, for power requirements for SOME applications/drives. One is for signal+power, one is power only and is not needed with many drives. – acejavelin Sep 25 '18 at 13:59
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Many computers explicitly follow the USB spec and will limit current per port to 500mA and have active protection to prevent over current situations.

In such a case your hard drive will not work without a second USB connection.

Many computers (and especially other media devices such as TVs) do not bother to limit the current out of their USB ports and so you will probably get away with only using one connection.

You apparently have one of the latter or have only connected drives that do not need more than 500mA.

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  • thanks! can you expand your answer a bit more: you say many computers with either case. can you point out a bit more specifically, what kind of computers (or specific brands? or laptops?) do limit their power? also, what kinds of hdd needs more than 500mA - 3.5 inchs? – T.Todua Sep 25 '18 at 14:16
  • All I can really say is that "it depends". You have to try it and see. There are many different 2.5" drives with different configurations of platters and motors. I've seen some that say 450mA, others with 700 to 800mA ratings. How computers are made vary a lot over time as well as manufacturers and there is no exhaustive list as things are being designed and changed all the time. – Mokubai♦ Sep 25 '18 at 14:24
  • For USB power negotiation, some just deliver whatever is demanded, but others only deliver after a device identifies itself as compliant with the manufacturer's safety specifications. For instance, my Samsung Galaxy Note II had a fast-charge ability from its charger, and my iMac at the time could output fast charging to my iPad, but the iMac could not fast-charge the Note. There are standards for USB power negotiation, but the manufacturers just treat them as guidelines. – Christopher Hostage Sep 25 '18 at 15:03
  • And sometimes, the device really does need so much power that regardless on how much the single USB connector can give, the device either fails, or limits to slower speeds with just one connector. – LPChip Sep 25 '18 at 15:43
  • Port power control is OPTIONAL in USB specifications. The only requirement is to provide AT LEAST 500 mA on USB 2.0 ports and 900 mA on USB 3.0 ports. So system developers have some freedom what to design for. Some PC have "ganged" power across groups of ports, protected only by a single resettable fuse of sufficient current capability. This is all within USB specifications. – Ale..chenski Sep 26 '18 at 4:32

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