1. A method of detecting if WORM data has been tampered with, the data having been recorded in a sequence of data sets, each of the data sets including a write pass value, and the write pass values having been incremented at intervals, the method comprising the steps of reading the data sets in sequence, comparing the write pass value of each data set with the write pass value of the preceding data set and indicating if the write pass value of any of the data sets differs from the write pass value of the preceding data set by an amount that breaches a predetermined rule, the predetermined rule providing that the write pass values are incremented each time a predetermined number of the data sets has occurred in the sequence.