Most of the time, problems like yours are caused by newer motherboards that only have one ide connector. The original xboxhdm expects your xbox harddisk on the primary ide as master and the cdrom on the secundary ide as master. If it's not connected that way, the linux os doesn't access the cdrom right, giving you the message that it can't find the cdrom.
Ldotsfan created a new xboxhdm 2.x that can boot from an usb stick, to avoid those problems.
Another option is locating an old pc somewhere with a mobo that still has 2 ide connectors. Annything PII or PIII should be able to do the job.
Voor de rest is het hartstikke eenvoudig.
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mvg.