Seagate launched new series of 2.5" external HD known as Replica.
Similarly to the Time Machine function of Mac OS X, those external HD are bundled with a software taking care of the entire backup of the PC. In case of crash of the computer, it is possible to recover the environment corresponding to the last backup. Seagate offer 2 models, one dedicated to single user (250 GB) and a larger unit (500 GB) able to manage backup of several user environment. Of course, the backup software is only compatible with Windows, but if you have Mac OS X 10.5, Time Machine does the same job independently of the manufacturer of your external storage unit. At the time we write those lines the price of such devices remains unknown