This site has some useful information regarding platters:http://yertech.blogspot.com/2009/08/35-hdd-platter-capacity-model-list.html
At least the following 500Gb models/revisions should be single platter:Caviar Black:
WD5001AALS revision 00E3A0Caviar Blue:
WD5000AAKS revisions 00M9A0/00D2B0/00V1A0/65V0A0Caviar Green:
WD5000AADS revisions 00S9B0/00L4B
To benefit from this information, however, you would need to know the drive's revision before purchasing. For example, some revisions of WD5001AALS still have 2x250 Gb platters.