Hi Maciorz,
I would suggest you to try the steps and check if it helps.
a) Right-click the USB drive or flash memory card that Windows is using for Ready Boost, then click "Properties."
b) Click the "Ready Boost" tab in the "Properties" window for the USB drive or memory card.
c) Select the radio button labeled "Do not use this device" and then click "OK." This step should automatically delete the Ready Boost cache on the device. If you are still unable to use the full capacity of the device, continue to the next step.
d) Double-click the USB drive or memory card.
e) Right-click the file "ReadyBoost.sfcache," then click "Delete."
f) Click "Yes" to delete the ReadyBoost cache.
Re-plug ready boost device which enables the ready boost service that will create new cache.
Hope this information helps. Reply to the post with updated status of the issue to assist you further.