cache creation failed

here is the error message which the system shows me when I am trying to create a cache.
cache creation failed: get_device for source device failed.

and here is the command which I used to create cache:

sudo eio_cli create -d /home/ubuntu/Downloads/story1 -s /home/ubuntu/Downloads/story2 -m wb -c my_cache

how should I solve this problem?
Many thanks.