http://windows.microsoft.com/en-us/windows7/Turn-ReadyBoost-on-or-off-for-a-storage-device
Note:
If your computer has a hard disk that uses solid-state drive (SSD) technology, you might not see an option to speed up your computer with ReadyBoost when you plug in a USB flash drive or flash memory card. You may instead receive the message, "Readyboost is not enabled on this computer because the system disk is fast enough that ReadyBoost is unlikely to provide any additional benefit." This is because some SSD drives are so fast they're unlikely to benefit from ReadyBoost.
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
- Source (Microsoft website)