(If you wish to have an answer to your question, white iPod drives spin at the same RPM as laptop drives - so they are usable for booting. Microdrives (used in the mini) spin much slower and thus would make for very slow booting/usage.

The Microdrive is also packed in a far more compact space, and so the heat generated would cause problems for neighbouring components when the drive is in use for long-ish periods of time).