according to the FAQs Drive Upgrade Guide its a 2.5mm hex for the 4 on the fascia
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-D Modifying and Tweaking is a journey, not a destination................................ MKIIa : 60gig - 040103286 - Blue - v2 + PCATS tuner MKIIa : 20gig - 040103260 - Blue - v3a8 + Mark Lord Special Edition Cherry Dock