From the Drive Upgrade Guide:

    Note that it must be the correct size jumper, which is 2mm. If you use the wrong size jumper, for example, if you use a 2.54mm jumper, you might get intermittent problems where the jumper loses contact because the grippers on the jumper are too large for the pins on the disk drive. According to Hugo, the correct jumpers are sold by Farnell, part numbers 510920 (blue), 510932 (black) and 510944 (red). For the US, the Jameco part number is reportedly 119263.
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Tony Fabris