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The first choice is size, physical size that is. The drive must be 2.5". This is the standard size for laptops. The only other size is 3.5" which are destined for non laptop PCs. Larger than 2.5" won't fit. Smaller ones are likely to be available as time goes by but for now it's just 2.5" to look for. By the way, don't bother taking a tape measure to the shop trying to measure 2.5" as this the the diameter of the internal disc that you won't be able to see. At some point you are going to open up your laptop and then you will see one for real and you won't then have to worry about measuring anything to see if it is 2.5"
By Paul E. Coughlin
SaneThinking.com
8 June 2008