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I think this may be one of those Dells where you can only see the top edge of the hard disk so if you aren't familiar with the setup, you might find it hard to spot. Look for a black metal part, about four inches by half an inch with a green or purple
plastic strip on each side. There's also a ribbon cable and a four pin power lead which you need to pull out. Squeeze those plastic strips and pull the disk upwards.