1 ) A method for creating efficient updates to a previously stored file, said method comprising: (a) receiving a first file comprising a first plurality of sequences of data; (b) receiving a second file comprising a second plurality of sequences of data; (c) creating a hash table having a plurality of entries, each of the plurality of entries corresponding to a respective one of the first plurality of sequences, and wherein at least two of said entries correspond to overlapping sequences of data; (d) computing hash values for said second plurality of sequences of data; (e) comparing each of the second plurality of sequences of data with sequences from the first plurality of sequences having the same hash value to determine sequences of data present in both files; (f) storing representations of lengths and locations of said sequences of data present in both the first and second files and (g) creating a third file comprising sequences of data from the second file and representations of locations and lengths of said sequences of data present in both the first and second files.