Hi,
this issue came up in this forum already. Apparantly in the other case, the harddisk was connected as master and the cdrom as slave on the same ide cable. Are you sure that your cdrom is connected to a different ide port on your motherboard? (A flatcable going from your computerboard to your cdrom, and nothing else ... and another flatcable going from the harddisk to your computerboard ...)
If everything is connected fine, and xboxhdm is unable to unlock your harddisk, than you might try the dostools.
Connect the harddisk as a slave disk to your pc. Start your windows OS and use the liveinfo program. Load the eeprom.bin file of your xbox. In the bottom fields, select the xbox harddisk. The program will give you the xbox password for that drive.
Shutdown the pc. Connect the xbox harddisk as master drive on the primary ide controller. Boot your pc from a dos bootable floppy and use the dos tools to unlock the harddisk. Just be very carefully with the lock program to make sure you don't accidently lock your harddisk with a password you can't remember afterwards.
regards.