Right click on the drive and select Properties. You should see a tab called ReadyBoost, where you can select the option to use the device as a ReadyBoost device.
Keep in mind that the device must have a certain speed. Vista will check the speed of the device and tell you it is not fast enough to be used for ReadyBoost if it doesn't meet the the speed requirements, which are:
2.5MB/sec throughput for 4K random reads and 1.75MB/sec throughput for 512K random reads. |