iMacIntel: "I do not like your HD"

A reader from XLR8Yourmac has decided to change the default HD of his brand new iMacIntel.
Looking for a larger storage capacity, he has decided to install a 400 GB Western Digital WD400KD.
The problem is that after hours of work, the disc was simply not recognized and refused to mount! He is still struggling with it.
So do not change your HD in your iMacIntel before the reason of this problem is identified and solved.