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