Hello *Threepio*,
I understand your interest in this mysterious rubber plug. After examining the plug, it seems like it was simply a design decision made in the process of creating the device. While removing it does allow access to a small part of the motherboard, it is unlikely that it was put there simply for that easy access as any operation that would require access to the motherboard would need more access than that. There seems to be no reason for it except design purposes; the rubber plug is a bit more interesting than a flat piece of plastic in that location. I apologize that it was nothing more interesting than that, but I thank you for the fun question. If you have any other questions, feel free to contact us again. You can find additional support at support forums at community.htc.com. There is also a customer satisfaction survey for you to take if you are interested.
Philip HTC Technical Support
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