1. A method of data progression in a disk drive system, the method comprising: determining a cost of operating for each of a plurality of data storage devices, each data storage device storing data in a plurality of data pages; classifying each of the plurality of data storage devices based on cost; at time intervals, generating one or more snapshot point-in-time copies (PITCs) of the data for the plurality of data storage devices, each snapshot PITC identifying changes to the data stored in one or more data pages that have taken place since a previous snapshot PITC taken at a previous time interval; checking the one or more snapshot PITCs to determine whether data stored in a data page of a data storage device of a first classification has been recently accessed; and if the data stored in a data page stored on the data storage device of the first classification has been recently accessed, moving the data to one or more data storage devices of a second classification.