Actually, don't all microdrives pull significantly more power than flash cards?
Though the new generation of microdrives (4GB+) is much better than the first generation (<4GB), peak current draw for all microdrives is significantly higher than their typical flash counterparts. But when you consider typical usage patterns, the difference becomes pretty inconsequential. Most Zaurus software makes very little use of storage when running. In fact the only category of applications I can think of that makes continuous use of storage when operating is a media player. And when you factor in caching and buffering, the number of physical reads from storage per unit time remains realatively low even in that case.
When the microdrive is not being actively read or written, it goes into a power saving mode where it spins down and draws very little current. CF cards are supposed to do the same but in my experience many have very poorly implemented "sleep" modes (and some are just completely broken). But because their idle power requirements are fairly modest, most people don't notice.
Now, if you decide to put an active swapfile on the microdrive or you use an app that does some continuous data logging, you may see some battery life impact. But I'm willing to bet that those types of activity are the exception rather than the norm. Most folks use microdrives for carrying large amounts of data (multimedia, documents, ebooks, etc.) that isn't continuously accessed.
As I've said before, I keep a 4GB Hitachi drive in my C760 95% of the time (the other 5% there's a WiFi or Bluetooth card in there). I used a 512MB Sandisk CF card before that in exactly the same manner and I've noticed no difference in overall battery life -- even when playing AVI and MP3 files.
