Readyboost cache disappears!

Hi all

I've just bought myself a Sandisk Cruzer Titanium 2GB for the sole purpose
of using it as a ReadyBoost device.
Vista detects the device, recognizes it as ReadyBoost compliant and allows
me to allocate a ReadBoost cache on it (so far so good).

Unfortunately, less than one minute after creating the cache file, Vista
decides to delete it. It doesn't matter how many times I recreate the
cache, reinsert the device (in any USB port) or reformat it - the readyboost
file disappears seconds after it has been created.

Has anyone else had this problem? Any advice for a frustrated user?

Thanks for nay help
Dave

Running Vista Ultimate RTM 32-bit with 1GB memory.