Disk buffer
When writing data to the disk, the data needs to be written to a certain position on the disk. This is done by a component called disk read/write head.  This head needs to be moved to (or near) the correct track. After some settling time the read head begins to pick up bits. Usually, the first sectors to be read are not the ones that have been requested by the operating system. The disk's embedded computer typically saves these unrequested sectors in the disk buffer, in case the operating system requests them later.