Without power management, disks on portable computers consume 20-30% of the system's power. Previous papers showed that spinning down a 1 watt drive after 3-5 minutes of inactivity reduces the average power consumption to 0.5 watts with a slight performance penalty. We explored the addition of an infinite non-prefetch cache to filter disk traffic and showed it to be ineffective at reducing disk power consumption or improving performance. Modifying the cache to perform whole-file prefetching gave the next step of improvement by reducing the disk power consumption to 0.1 watts while retaining current performance levels. The traditional file system performance model focuses on cache hit rate as the performance determinant. This paper introduces a new performance model for systems that spindown disks and shows that performance is no longer a function of hit rates, but rather of read-interrequest times and disk spindown delays. Drive durability is an issue for low power file systems because of the friction induced wear during disk spinup. Periodic writes, which result in periodic disk spinups, will very likely lead to premature drive failure. We show that approximately 1 MB of flash memory used as a write buffer eliminates the need to spinup the disk for writes.