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Thanks for both replies. The Crucial site is extremely good and I looked up their own drives,not rated too highly but cheap at £85 for the 240Gb,but a bracket is needed for holding the 2.5" drive into the 3.5" bay! another £20. Apparently if I do a Macrium backup to another drive,then swap the drives and then do a Macrium Restore onto the new drive,everything should be fine!!!
Afraid I'm with Old_Geezer on this one! But, I shall keep watching.
Thanks a lot folks.