ZzZzZzZzZzZzyyeeooowww!!
If you only have multimedia files on your CF microdrive and no applications (which I wouldn't recommend), then your MD should power down while it is not accessed. Certain multimedia players, like TCPMP (
http://tcpmp.corecodec.org) have a "microdrive mode" which will buffer your files and allow the MD to go dormant, saving battery.