CacheRoot /var/cache/apache2/mc/mod_cache_disk
CacheQuickHandler on
CacheLock on
CacheLockPath /tmp/mod_cache-lock
CacheLockMaxAge 5
CacheIgnoreHeaders Set-Cookie

CacheEnable disk /
CacheHeader on
CacheDefaultExpire 800
CacheMaxExpire 64000
CacheIgnoreNoLastMod On
ExpiresActive on
ExpiresDefault A300

CacheRoot /var/cache/apache2/mc/mod_cache_disk.  mc is for the mc.scsiraidguru.com web site. 

I set a custom path for each of my virtual hosts.   The /var directory has its own virtual disk.