cache_utils.group_backend
is a cache backend that stores data in memcached. By itself it doesn't store it anywhere - on C
drive or anywhere else. Memcached also doesn't keep its data on disk, but in memory (RAM).
As a consequence there is no right answer to your question.
If you want a cache backend that keeps its data on disk use FileBasedCache, but keep in mind that its much slower than memcached.