Mechanical hard drives can be very fast when reading sequential files, such as copying movies. But when we usually use the operating system, the most common application is reading scattered files, such as operating system startup, opening programs, web browsing cache, etc. Each time we need to read many small files distributed in different locations on the disk. Mechanical hard drives rely on the magnetic head to constantly seek before it can read. It seeks and reads again and again, so a lot of time is spent on seeking and reading scattered small files.