You can use either a 2.5" or a 3.4" external drive in either a desktop or a laptop. With external use the size makes no difference. it is the interface it is connected to that matters and that is usually USB which is the same on both desktops and laptops..
There is no difference in functionality between a 7200 rpm and 5400 rmp disk except that the drive spindle on a 7200 rmp disk spins faster and therefore is likely to be able to transfer data faster.
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