Files are linked lists of blocks. The sector header of each sector on the hard disk contains pointers to the previous and next blocks in the file, along with some other info that I've forgotten Files may be sparse - the fact that block n exists does not mean that block n-1 does Block 0 of each file (I think, maybe block -1) contains the 'file descriptor' - bascially an i-node. Negative block numbers are the file allocation map. You can use this to quickly find any block in the file without following the links.