you can get a 250 seagate st3250820a from scan.co.uk first download seagate discwizard from seagate. when you get the new drive you install it as slave. follow the discwizard instructions and it will copy the entire contents of your drive over to the new drive.
when all is done remove the faulty drive and set the new drive to master.
the discwizard prints all the stages of the instructions out before you start.
I have used this and all went ok.