FAT file allocation table PCs store and retrieve files on hard disks using this filing system, which divides the hard disk into "clusters" of bytes and then files data into these clusters. When a program calls for a file, the FAT looks up the locations of all the clusters where the data is stored. The cluster size depends on the size of the hard disk, with bigger drives using bigger clusters. Here's the tricky part: a single cluster can only store data from a single application or file; if the data doesn't fill the whole cluster, then the rest of that hard disk drive space is wasted. Let's say a cluster is 32K; a 42K word processing document would take up a full cluster, plus 10K of a second. The 22K remaining in the second cluster remain unused, no matter how full the hard disk appears to be.