Yes; power is not supplied by the data cable.
An exception might be a small-form-factor, external, USB-connected drive, as the USB port can power low-power devices. If an external USB hub is expected to power external devices, it's best if the hub itself is powered, otherwise available power is split among too many devices, potentially leaving something underpowered.
Clarifying the question. Currently running WinXPPro in this computer, which is not mine, I want to connect my HHD (350gig) temporaly and be able to choose from which to boot. My HHD as XP Home, will it affect my lincense from Microsoft?
It means a Hard Disk Drive that has 650 Megabytes of storage on it.
AnswerThere is air in between hdd platters. When they are spinning, it is only a thin layer of air that keeps them separated. The gap between both platters is so small that a dust particle can cause a jam. That's why, if you ever see some really specialised data recovery experts they have to open the HD in a "clean room", otherwise there is a good chance a peice of dust, hair etc will get into that little gap where the air is. I read too, that even if you touch the platter with your bare hand, the fingerprint left on the platter will cause a jam. That's how small the gap is.
In their basic form, floppy discs are common hard disks are the same technology. They both use magnetism to store and read data. They both use a spinning platter read by a horizontally moving read head.
The key differences are that hard disks have the data and reader in the same package, this is why hard discs have evolved in storage capacity while floppy disks remained the same. It was necessary for the written layer of a floppy disk to have the same data density and dimensions as they had to be compatible with floppy disk drives. However with hard discs the drives and read heads could evolve along side the data layer.
Hard discs now have metal platers, and store data at a much higher density. The speed of the mechanisms are much much faster. New technologies such as perpendicular storage can be added and have no impact on compatibility.