Small form factor hard disk drives (HDDs) are proliferating into embedded systems and being used for numerous tasks, like storing map data for GPS systems, pictures for digital cameras and video streams for PMP. A typical HDD datasheet gives the maximum burst transfer rate, but effective system design requires knowing other parameters, such as achievable sustainable performance, CPU impact, and energy required. This paper covers major factors affecting real-world system performance, why they are important, how to measure or calculate them, the design/software/hardware tradeoffs inherent in them, and illustrates concepts with data collected from HDDs connected to Intel PXA processors.