What is ReadyBoost?
It is a fast disk cache which can help increase performance in low specification systems. It was developed for use in Vista, and has carried over into Windows 7 and 8.
Devices suitable for ReadyBoost
Fast USB flash drives. If your flash drive does NOT meet the requirements set by Microsoft, you will NOT be offered the ReadyBoost option. You will also not be offered the option if your computer has an SSD drive fitted either as the boot drive or secondary drive.
Will ReadyBoost work with Windows 8?
Windows 8 will present you with this message if it doesn’t think that ReadyBoost would help. Remember that these limits are set Microsoft:
"This device cannot be used for ReadyBoost. ReadyBoost is not enabled because this computer is fast enough that ReadyBoost is unlikely to provide additional benefit."
Now, you may think that Microsoft is being a little heavy handed, but somebody has to save you and a perfectly good flash drive from your outmoded beliefs..