Trying to know what is the best of my flash memories to use ReadyBoost I realize that I don't know how much of fast flash memory each of my flash drives have. One can read:

In some situations, you might not be able to use all of the memory on your device to speed up your computer. For example, some flash memory devices contain both slow and fast flash memory, but ReadyBoost can only use fast flash memory to speed up your computer.

From http://windows.microsoft.com/is-IS/windows7/Using-memory-in-your-storage-device-to-speed-up-your-computer