Drive rotation speed is not the only thing that influences data transfer speeds. The drive cache size and caching algoritms are important as well.
On USB 2.0, the usb speed is usually the speed limiting factor. An usb port should be able to provide 0.5 Amps to a connected device.
The power consumption of most 2.5" drives fluctuates around this. If a device has multiple usb ports, usually the supply is simply shared by the different
ports. As long as the total requested current consumed on both ports doesn't exceed "0.5 Amps * the number of ports".
In case of the Wii, this means that the total current can be up to 1 Amp.
The most power hungry item in a harddisk is the platter motor. At startup, that motor can create a small power peak.
7200RPM motors consume more power than 5400RPM motors. (In case people might wonder why there still are 5400RPM harddisks)
A bus powered USB hub only provides 0.1 Amp power on it's downstream ports.
The idea is to have 0.1 Amps for the hub electronics and 0.1 Amp for 1 of the 4 downstream ports, bringing the total again to 0.5 Amps.
A 3.5" harddisk has it's own power adapter. If it goes into suspend after a period of inactivity (normally the motor is switched off), it can give problems
on the wii because it takes time to resume. If it doesn't, you will need to remove the power yourself every time you turn the system off. Some disks will
power down if the usb power dissapears (like in case of the wii). On a pc this doesn't always work. Some pc's leave the usb power on, so that usb keyboards can keep working, giving them a way to turn the pc on.
A 2.5" harddisk can have an annoying suspend feature as well. As those are meant to be connected to a laptop, more power consumption is an important feature to enhanced battery life.
http://gbatemp.net/t280043-joyflow-a-mod-o...low?&st=420
Here Xflak describes the Seagate Goflex drives. It's a 2.5" drive with a removable usb interface. You can buy a usb 3.0 interface for it if you need the speed and your pc supports usb 3.0, and you can buy it with the cheaper usb 2.0 interface for wii interfacing.
I am no representative of seagate, and I don't use that system myself, but I did have my part of misery finding a compatible drive for the wii.
If Xflak says that these work very well, I believe him....