Vista's ReadyBoost flash drives lack significant boost

[freshseasons]

I assume we can use a portable usb hdd too for doing this, is that right?? or it has to be a flash drive?

I think the flash drive is preferable.
The USB harddisk will not be able to pass the ready boost test when Vista asks for in when you plug it.Moreover there is no significant boost from the harddisk.
The Very purpose of ReadyBoost is taking those files of slow harddisk.
So it has to be Flash Drive.